Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
Thank you for your work! I just glanced over it but I'll suggest it to be linked to from the homepage of my university's functional programming course. However, thirteen pages can hardly be called cheatsheet. It's more like a quick reference. You could add [100,99..] infinite liste of numbers from 100 downwards to you numbers section, as it is an example where the range does go backward. Adrian Am 11.10.2008 um 01:08 schrieb Justin Bailey: All, I've created a cheat sheet for Haskell. It's a PDF that tries to summarize Haskell 98's syntax, keywords and other language elements. It's currently available on hackage[1]. Once downloaded, unpack the archive and you'll see the PDF. A literate source file is also included. If you install with cabal install cheatsheet, run cheatsheet afterwards and the program will tell you where the PDF is located. The audience for this document is beginning to intermediate Haskell programmers. I found it difficult to look up some of the less-used syntax and other language stumbling blocks as I learned Haskell over the last few years, so I hope this document can help others in the future. This is a beta release (which is why I've limited the audience by using hackage) to get feedback before distributing the PDF to a wider audience. With that in mind, I welcome your comments or patches[2]. Justin [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ CheatSheet [2] git://github.com/m4dc4p/cheatsheet.git ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe PGP.sig Description: Signierter Teil der Nachricht ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to translate Haskell to other languages?
Hello,
I was thinking about translating Haskell to other languages, python being
the main one at the moment.
Here is my attempt at manually encoding Haskell in Python:
\begin{code}
import types
class thunk:
'''Thunks allow us to delay a computation and they also store their
value inside themselves once they have been accessed.'''
def __init__(self, v):
self.v = v
def value(self):
'''Force the thunk to be calculated by referencing it.'''
while self.isReducible():
self.reduce()
return self.v
def reduce(self):
'''Reduces the thunk, by either calling the represented function
or reducing the layers of thunk.'''
if (type(self.v) == types.FunctionType):
self.v = self.v()
else:
self.v = self.v.value()
return self.v
def isReducible(self):
'''Returns True when the thunk is still callable.'''
return isinstance(self.v, thunk) or \
type(self.v) == types.FunctionType
class nil:
'''Empty list element'''
def __init__(self):
pass
class cons:
'''Non-empty lists'''
def __init__(self, head, tail):
self.head = head
self.tail = tail
'''Unpack the cons cell'''
def uncons(self):
return self.head, self.tail
def htail(t):
'''This function works like Haskell's tail function.'''
l = t.value()
x, xs = l.uncons()
return xs
def plus(t1, t2):
'''Adds numbers'''
i1 = t1.value()
i2 = t2.value()
return thunk(i1+i2)
def zipWith(f, t1, t2):
'''This is like Haskell's zipWith function.'''
l1 = t1.value()
if isinstance(l1, nil): return thunk(nil())
l2 = t2.value()
if isinstance(l2, nil): return thunk(nil())
x, xs = l1.uncons()
y, ys = l2.uncons()
zw = thunk(lambda: zipWith(f, xs, ys))
fxy = thunk(lambda: f(x,y))
return thunk(cons(fxy, zw))
def fibs():
'''This is the classic fibs:
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)'''
f1 = thunk(1)
f2 = thunk(1)
fn = thunk(fibs)
rest = thunk(lambda: zipWith(plus, fn, htail(fn)))
restlist = thunk(cons(f2, rest))
fiblist = thunk(cons(f1, restlist))
return fiblist
def hmap(f, t):
'''map _ [] = []
map f (x:xs) = f x : map f xs'''
l = t.value()
if isinstance(l, nil): return thunk(nil())
x, xs = l.uncons()
fx = thunk(lambda: f(x))
mapfxs = thunk(lambda: hmap(f, xs))
return thunk(cons(fx, mapfxs))
def show(t):
'''show :: a - String'''
v = t.value()
return thunk(str(v))
def printList(t):
'''This just gives us a way to debug lists.'''
v = t.value()
print [,
while True:
h,t = v.uncons()
print %s % h.value(),
if isinstance(t.value(), nil): break
else: print , ,
v = t.value()
print ]
def take(tn, tl):
'''take n _ | n = 0 = []
take _ [] = []
take n (x:xs) = x : take (n-1) xs'''
n = tn.value()
if n = 0: return thunk(nil())
l = tl.value()
if isinstance(l, nil): return thunk(nil())
x,xs = l.uncons()
nminusone = thunk(lambda: plus(tn, thunk(-1)))
takerec = thunk(lambda: take(nminusone, xs))
return thunk(cons(x, takerec))
\end{code}
You can try this out in python with:
tenfibs = take(thunk(10), fibs())
printList(tenfibs)
This will print the first 10 fibs.
Questions:
I think the examples above are correctly lazy. Have I missed something?
I noticed my thunks can get wrapped in each other, is this to be expected,
or am I doing it wrong?
Is there an easier encoding using generators? When I started I was using
generators instead of thunk, but I found it was complicating my design so I
removed it. And yet, since generators are python's version of thunks, it
seems like there should be a more natural encoding there.
I'm not explicitly using a graph reduction algorithm to reach WHNF, does
this mean my translation is wrong?
Are there some well known test cases I should try? Anyone know of a paper
that discusses making this translation?
I am trying to avoid writing an interpreter in Python for Haskell. My goal
is to translate Haskell functions into the equivalent Python. I'm also
hoping to avoid needing a G-machine.
Thanks!
Jason
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Re: [Haskell-cafe] Haskell in Artificial Intelligence
On 11 Οκτ 2008, at 10:57 ΠΜ, Krasimir Angelov wrote: Hi Christos We use Haskell for natural language processing. See here: http://www.cs.chalmers.se/~aarne/GF/ Best regards, Krasimir Thank you very much for your response Krasimir. Best regards, Christos On Fri, Oct 10, 2008 at 8:45 PM, Christos Chryssochoidis [EMAIL PROTECTED] wrote: Greetings, I'm interested in doing a survey about the use of Haskell in the field of Artificial Intelligence. I searched in Google, and found in the HaskellWiki, at www.haskell.org/haskellwiki/Haskell_in_industry, two organizations that use Haskell and do work related to AI. Besides that, I haven't found much else. Could somebody from the Haskell community give me some pointer to a project or system related to AI that uses Haskell? Thank you very much in advance. - CC ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell in Artificial Intelligence
Hi CC, you can find a word about possible usage of functional programming in AI in following paper: http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html . Besides in HackageDB you can find separate category AI. I had pleasure also to write an AI of simple game (http://en.wikipedia.org/wiki/Paper_Soccer): http://hackage.haskell.org/cgi-bin/hackage-scripts/package/foo . Currently I'm working on AI of another game: http://www.educationallearninggames.com/how-to-play-pylos-game-rules.asp . It's not much though. rgds, Bartek On Friday 10 October 2008 20:45:43 Christos Chryssochoidis wrote: Greetings, I'm interested in doing a survey about the use of Haskell in the field of Artificial Intelligence. I searched in Google, and found in the HaskellWiki, at www.haskell.org/haskellwiki/Haskell_in_industry, two organizations that use Haskell and do work related to AI. Besides that, I haven't found much else. Could somebody from the Haskell community give me some pointer to a project or system related to AI that uses Haskell? Thank you very much in advance. - CC ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
Very nice!
I have my own cheat list, which are haskell commands I find useful but
find inconvenient or difficult to look up in the supplied
documentation. I actually hardwire my cheats into .bashrc doing
something like
thartman_haskell_cheatting() {
cat EOF
blah blah
cheat
}
so i can quickly see all my haskell cheats using tab completion. but a
pdf is even nicer :)
**
[EMAIL PROTECTED]:~/Desktopthartman_haskell_oneliners
ghc -e '1+2'
[EMAIL PROTECTED]:~/Desktopthartman_haskell_regex_hints
cabal install pcre-regex
Most likely want:
Prelude Text.Regex.PCRE user123 =~ ^(user)(\d*)$ ::
(String,String,String,[String])
(,user123,,[user,123])
That is: (before match, match, after match, subgroups)
or maybe
Prelude Text.Regex.PCRE user123 user456 =~ (u(se)r)(\d*) :: [[String]]
[[user123,user,se,123],[user456,user,se,456]]
if you need all submatches of all matches
since
Prelude Text.Regex.PCRE user123 user456 =~ (user)(\d*) ::
(String,String,String,[String])
(,user123, user456,[user,123])
doesn't quite do what I want -- no submatches
and there's no instance for :: (String,String,String,[String])
I don't need all submatches of all matches very often though.
:: Bool -- did it match
:: String -- first match
:: [String] -- every match
:: :: (String,String,String) -- before, matched, after
http://www.serpentine.com/blog/2007/02/27/a-haskell-regular-expression-tutorial/
[EMAIL PROTECTED]:~/Desktopthartman_haskell_testing_things
import Data.Test.HUnit
runTestTT $ TestCase $ assertEqual meh 1 2
runTestTT $ TestList [ TestCase $ assertEqual meh 1 2 ]
[EMAIL PROTECTED]:~/Desktopthartman_haskell_hints
offline documentation:
ghc-pkg describe bytestring | grep -i doc
or probably just
haddock-interfaces:
/usr/local/share/doc/ghc/libraries/bytestring/bytestring.haddock
haddock-html: /usr/local/share/doc/ghc/libraries/bytestring
note to self:
start using cabal install --global (or whatever the flag is)
so all documentation is browsable from one place
Use language pragmas, with commas
And you can't put LANGUAGE and OPTIONS_GHC in the same pragma
{-# LANGUAGE NoMonomorphismRestriction, PatternSignatures #-}
{-# OPTIONS -fglasgow-exts #-}
Debugging
toVal.hs:30:17:
Couldn't match expected type 'blee'
against inferred type 'bleh'
bleh is whatever is at 30:17
blee is something that's wanted by whatever is calling the value at 30:17
If the error is in the definition of some function,
then probably one function case conflicts with another, you can
ignore other functions.
In this case you will only get one line:col to look at.
If there are more than one line:col to look at, possibly separate
functions are in conflict.
So, smart to always fix in the definition of type errors first.
Still baffled? Won't compile?
Give top-level functions type signatures. Won't hurt, might help.
:set -fwarn-missing-signatures
or {-# OPTIONS -fwarn-missing-signatures #-}
Start commenting out calling functions until it compiles, and then
look at the signatures.
And then type the signatures in explicitly... does something look funny?
Like, wrong number of args? Maybe currying went wrong.
tag and bundle a distribution:
darcs tag 0.2
cabal configure
cabal sdist
cd dist; unzip, verify install does the right thing
http://hackage.haskell.org/packages/upload.html
check upload, and upload.
see also http://en.wikibooks.org/wiki/Haskell/Packaging
group module imports from multiple modules in one place:
module MyInductiveGraph (
module Data.Graph.Inductive,
module EnoughFlow
)
where
import Data.Graph.Inductive
import EnoughFlow
**
2008/10/11 Justin Bailey [EMAIL PROTECTED]:
All,
I've created a cheat sheet for Haskell. It's a PDF that tries to
summarize Haskell 98's syntax, keywords and other language elements.
It's currently available on hackage[1]. Once downloaded, unpack the
archive and you'll see the PDF. A literate source file is also
included.
If you install with cabal install cheatsheet, run cheatsheet
afterwards and the program will tell you where the PDF is located.
The audience for this document is beginning to intermediate Haskell
programmers. I found it difficult to look up some of the less-used
syntax and other language stumbling blocks as I learned Haskell over
the last few years, so I hope this document can help others in the
future.
This is a beta release (which is why I've limited the audience by
using hackage) to get feedback before distributing the PDF to a wider
audience. With that in mind, I welcome your comments or patches[2].
Justin
[1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CheatSheet
[2] git://github.com/m4dc4p/cheatsheet.git
Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
On Saturday 11 October 2008 01:08:15 Justin Bailey wrote: This is a beta release (which is why I've limited the audience by using hackage) to get feedback before distributing the PDF to a wider audience. With that in mind, I welcome your comments or patches[2]. Justin [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CheatSheet [2] git://github.com/m4dc4p/cheatsheet.git Looks good! Some minor issues: - sometimes it is a bit verbose for a cheatsheet; for example the reference to language C in the section about the layout rule. - in the section about strings, you give an example of a syntax error; it would be sufficient to show what is right. - in the section about operator precedence you define 'div1' to be right associative. Instead, you could show where right associativity is actually useful (=, ...). - The explanation of the layout rule is wrong. If you define more than one value in a let declaration, then it is only required that the identifiers start on the same column. - When I started to learn Haskell, I had problems with the use of (.) and ($). I had learned what function application and lambda abstractions look like, but then I looked at Haskell code written by experienced Haskellers and found expressions like (map (succ . succ) $ 1:xs) that I did not understand. A small section describing how to read such expressions could be useful for beginners. - the section about do-notation is more a mini-tutorial than a cheatsheet. Instead, you could show two or three examples that demonstrate how do- notation, list comprehensions and the operator = relate. That is what I had to look up more than once until I got used to it. There is also an example of what is wrong, where showing the right thing would have sufficed. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Interesting new user perspective
If you want quick examples of idiomatic haskell including stdin/stdout I/O, I like this page: http://www.haskell.org/haskellwiki/Simple_unix_tools -- ryan On Fri, Oct 10, 2008 at 7:08 PM, Iain Barnett [EMAIL PROTECTED] wrote: On 9 Oct 2008, at 9:33 pm, Andrew Coppin wrote: I think it's just the teaching of the language that needs work, not so much the language itself. As a newer user myself, I'd agree with this statement. I'd like to see far more mundane tasks solved in tutorials. The number of times building a parser or generating prime number is used as an example is out of proportion to the times you'd use these things[1]. Just simple, *really* easy things would be better. Maybe it's just me, but if I wanted to learn perl or ruby or python or C# I'm not sure I'd ever see a _tutorial_ containing a prime number. Haskell is can obviously do some really interesting things, but constantly having wikipedia open so I can look up whatever mathematical doodah has just been mentioned can get draining. Even Real World Haskell suffers a bit from this. Iain [1] In years of programming (other languages) I've never had to generate my own primes or build a compiler or a parser. I may have parsed things, but that's different to building an entire parser, if you get my drift. Actually, tell a lie. I have built a parser, but it's still not stuff for a beginner's tutorial IMHO. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
On Fri, 10 Oct 2008, Justin Bailey wrote: I've created a cheat sheet for Haskell. It's a PDF that tries to summarize Haskell 98's syntax, keywords and other language elements. It's currently available on hackage[1]. Once downloaded, unpack the archive and you'll see the PDF. A literate source file is also included. .. The audience for this document is beginning to intermediate Haskell programmers. I found it difficult to look up some of the less-used syntax and other language stumbling blocks as I learned Haskell over the last few years, so I hope this document can help others in the future. .. Justin [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CheatSheet [2] git://github.com/m4dc4p/cheatsheet.git ___ Justin (and everyone now contributing to this), thanks. This type of thing is very helpful. There is a well-known one-page cheat-sheet like this for Perl5 (and a newer one for Perl6 too). You can see it by typing `perldoc perlcheat` on a system with Perl. I wonder if we don't need something like that. On a related note, I had come up with this short list to help with how to read the Haskell symbols. I email this out when trying to get new people started: - Haskell symbology One thing that I found very difficult at first was how to read a lot of the symbols in the Haskell code. I made up a small cheat-sheet that explains how to read some of it: Some definitions of Haskell symbols: :: has type - to The type 'Integer - Integer' takes Integer as an argument and evaluates to an Integer. We say this type is Integer to Integer = evaluates or reduces to Used in function type signatures, read something like this: foo :: (Bar b) = b - c For all instances of type class Bar (here referred to as b), foo is a function from b to c. Or foo has type b to c. [a] list of a The family of types consisting of, for every type a, the type of lists of a. : cons List cons operator, adds first argument to the front of second, part of the List monad: (:) :: a - [a] - [a] Read: cons has type a to list of a to list of a !! List index operator [ 1, 2, 3 ] !! 1 = 2 (lists are 0-based) (!!) :: [a] - Int - a Read: (!!) has type list of a to Int to a | such that - drawn from = bind Part of class Monad then Part of class Monad - -- Dino Morelli email: [EMAIL PROTECTED] web: http://ui3.info/d/ irc: dino- pubkey: http://ui3.info/d/dino-4AA4F02D-pub.gpg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell in Artificial Intelligence
On 11 Οκτ 2008, at 11:41 ΠΜ, Bartosz Wójcik wrote: Hi CC, you can find a word about possible usage of functional programming in AI in following paper: http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html . Besides in HackageDB you can find separate category AI. I had pleasure also to write an AI of simple game (http://en.wikipedia.org/wiki/Paper_Soccer): http://hackage.haskell.org/cgi-bin/hackage-scripts/package/foo . Currently I'm working on AI of another game: http://www.educationallearninggames.com/how-to-play-pylos-game-rules.asp . It's not much though. rgds, Bartek Thanks very much for the very interesting pointers! Best regards, Christos On Friday 10 October 2008 20:45:43 Christos Chryssochoidis wrote: Greetings, I'm interested in doing a survey about the use of Haskell in the field of Artificial Intelligence. I searched in Google, and found in the HaskellWiki, at www.haskell.org/haskellwiki/Haskell_in_industry, two organizations that use Haskell and do work related to AI. Besides that, I haven't found much else. Could somebody from the Haskell community give me some pointer to a project or system related to AI that uses Haskell? Thank you very much in advance. - CC ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] External Sort and unsafeInterleaveIO
I kinda-sorta half-cabalized it at darcs get http://darcsdump.dreamhosters.com/external-sort (untested via cabal install but mostly done) As soon as my project gets approved I'll put it up on hackage. If Ben wants it under his account at hackage of course I'll defer to him. Thomas. 2007/7/18 Donald Bruce Stewart [EMAIL PROTECTED]: midfield: hi -- thanks for the useful comments! i will definitely go through them carefully. unfortunately for this code (but fortunately for me) i defend my dissertation on monday so i'm a little distracted right now. i'm more than happy to donate this code or whatever improvements happen to it. actually, hGetContentsWithCursor seems like a candidate for inclusion with Data.ByteStrings or Data.Binary or something -- it seems like it might find other uses. (i think you liked that bit of code because i ripped it off of you guys! it's very short hamming Can't fault that style ;) distance from the original.) anyhow, all that will have to wait a couple weeks or so. also i've never cabalized anything so i may come begging for help. We have a tutorial for that, luckily: http://haskell.org/haskellwiki/How_to_write_a_Haskell_program And a tool to automate it, mkcabal, so should be fairly straightforward. at some point i thought i saw how to do recursive external sort, to keep memory usage truly constant, but with my current lack of sleep i have lost that illusion. i'm also curious about the performance characteristics of this vs Prelude sort vs the version using the tournament mergesort apfelmus suggested. i need to find a computer with a lot more RAM than my weakling laptop. finally, it would be good to be able to have the blocksize controlled by Kb of RAM rather than # of elements, not sure how to get that information. ultimately this was part of my project to write lucene for haskell. i think with this out of the way, plus all the Data.Binary / ByteString goodness, it shouldn't take too long. keep writing good libraries for me! Great. Good to see things working. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell on the JVM
Are there, or have there been Haskell ports to the JVM? Are any of them alive and well? Is there an interest in hosting GHC on the JVM (besides my own). I know I don't have time to tackle such a project but I'd be extremely interested in using such a thing. Especially in my day to day work. I currently do a lot of Java - Erlang via JInterface, and have been for months, yet there's many tasks I could do better in a language like Haskell. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to translate Haskell to other languages?
Hi Jason, I don't know Python, but let me share some thoughts that you might find useful. First, a few questions about your manual translations. Are your functions curried? For example, can I partially apply zipWith? Also, you put a thunk around things like cons(...) --- should it not be the arguments to cons that are thunked? Now, on to an automatic translation. As you may know already, Haskell programs can be transformed to combinator programs which are quite simple and easy to work with. Here is what I mean by a combinator program: p ::= d*(a program is a list of combinator definitions) d ::= c v* = e (combinator definition) e ::= e e (application) | v (variable/argument) | c (constant: integer literal, combinator name, etc.) As an example of a combinator program, here is one that reverses the list [0,1,2]. rev v acc = v acc (rev2 acc) rev2 acc x xs = rev xs (cons x acc) cons x xs n c = c x xs nil n c = n main = rev (cons 0 (cons 1 (cons 2 nil))) nil This program does not type-check in Haskell! But Python, being dynamically typed, doesn't suffer from this problem. :-) A translation scheme, D[], from a combinator definition to a Python definition might look as follows. D[c v* = e] = def c() : return (lambda v1: ... lambda vn: E[e]) E[e0 e1] = E[e0] (E[e1]) E[v] = v E[c] = c() Here is the result of (manually) applying D to the list-reversing program. def nil() : return (lambda n: lambda c: n) def cons() : return (lambda x: lambda xs: lambda n: lambda c: c(x)(xs)) def rev2() : return (lambda acc: lambda x: lambda xs: rev()(xs)(cons()(x)(acc))) def rev() : return (lambda v: lambda acc: v(acc)(rev2()(acc))) def main() : return (rev() (cons()(0)( cons()(1)( cons()(2)( nil()(nil())) The result of main() is a partially-applied function, which python won't display. But using the helper def list(f) : return (f([])(lambda x: lambda xs: [x] + list(xs))) we can see the result of main(): list(main()) [2, 1, 0] Of course, Python is a strict language, so we have lost Haskell's non-strictness during the translation. However, there exists a transformation which, no matter how a combinator program is evaluated (strictly, non-strictly, or lazily), the result will be just as if it had been evaluated non-strictly. Let's call it N, for Non-strict or call-by-Name. N[e0 e1] = N[e0] (\x. N[e1]) N[v] = v (\x. x) N[f] = f I've cheekily introduced lambdas on the RHS here --- they are not valid combinator expressions! But since Python supports lambdas, this is not a big worry. NOTE 1: We can't remove the lambdas above by introducing combinators because the arguments to the combinator would be evaluated and that would defeat the purpose of the transformation! NOTE 2: i could be replaced with anything above --- it is never actually inspected. For the sake of interest, there is also a dual transformation which gives a program that enforces strict evaluation, no matter how it is evaluated. Let's call it S for Strict. S[e0 e1]= \k. S[e0] (\f. S[e1] (\x. k (f x))) S[v]= \k. k v S[f]= \k. k f I believe this is commonly referred to as the CPS (continuation-passing style) transformation. Now, non-strict evaluation is all very well, but what we really want is lazy evaluation. Let's take the N transformation, rename it to L for Lazy, and indulge in a side-effecting reference, ML style. L[e0 e1] = L[e0] (let r = ref None in \x. match !r with None - let b = L[e1] in r := Some b ; b | Some b - b) L[v] = v (\x. x) L[f] = f I don't know enough to define L w.r.t Python. I haven't tried too hard to fully understand your translation, and likewise, you may not try to fully understand mine! But I thought I'd share my view, and hope that it might be useful (and correct!) in some way. Matthew. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
On Fri, Oct 10, 2008 at 7:08 PM, Justin Bailey [EMAIL PROTECTED] wrote: I've created a cheat sheet for Haskell. It's a PDF that tries to summarize Haskell 98's syntax, keywords and other language elements. It's currently available on hackage[1]. Once downloaded, unpack the archive and you'll see the PDF. A literate source file is also included. It looks very nice, if a bit verbose. One minor comment is that on page 4 you give a type signature for if-then-else. I would contend that it should be Bool - a - a - a, instead. steve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on the JVM
LamdaVM was the only full-fledged effort to port Haskell to the JVM, and like most graduate school projects, the project is now dead. I have strong interest in hosting GHC on the JVM. And I suspect it would be good for the Haskell community, as the JVM already runs on nearly every machine known to man, has a wealth of cross-platform libraries, and is getting improved support for dynamic and functional languages (method handles, tail call). Regards, John A. De Goes N-BRAIN, Inc. http://www.n-brain.net [n minds are better than n-1] On Oct 11, 2008, at 10:07 AM, David Leimbach wrote: Are there, or have there been Haskell ports to the JVM? Are any of them alive and well? Is there an interest in hosting GHC on the JVM (besides my own). I know I don't have time to tackle such a project but I'd be extremely interested in using such a thing. Especially in my day to day work. I currently do a lot of Java - Erlang via JInterface, and have been for months, yet there's many tasks I could do better in a language like Haskell. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on the JVM
On 2008 Oct 11, at 12:07, David Leimbach wrote: Are there, or have there been Haskell ports to the JVM? Are any of them alive and well? YHC, last I heard, was alive and well and there's a YHC Core backend for the JVM. Is there an interest in hosting GHC on the JVM (besides my own). There's interest but my understanding is that the GHC backend architecture is not at all friendly to work with. That said, I hear in the next release (I think 6.12, not the 6.10 that's in beta) will have a redesigned backend architecture that is supposed to be much easier to work with, which will make it easier to provide native code generators for many currently poorly-served platforms --- including the JVM if anyone is so inclined. You might want to ask on glasgow-haskell-users to get the people who really know. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on the JVM
There's a YHC that can compile to JavaScript, and JavaScript can be run on Java... Which means, practically speaking, there is no YHC backend for the JVM. Regards, John A. De Goes N-BRAIN, Inc. http://www.n-brain.net [n minds are better than n-1] On Oct 11, 2008, at 11:08 AM, Brandon S. Allbery KF8NH wrote: On 2008 Oct 11, at 12:07, David Leimbach wrote: Are there, or have there been Haskell ports to the JVM? Are any of them alive and well? YHC, last I heard, was alive and well and there's a YHC Core backend for the JVM. Is there an interest in hosting GHC on the JVM (besides my own). There's interest but my understanding is that the GHC backend architecture is not at all friendly to work with. That said, I hear in the next release (I think 6.12, not the 6.10 that's in beta) will have a redesigned backend architecture that is supposed to be much easier to work with, which will make it easier to provide native code generators for many currently poorly-served platforms --- including the JVM if anyone is so inclined. You might want to ask on glasgow-haskell-users to get the people who really know. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon university KF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Interesting new user perspective
Andrew Coppin wrote: Martin DeMello wrote: http://blog.moertel.com/articles/2006/10/18/a-type-based-solution-to-the-strings-problem is a brilliant example of a common workaday problem found in other languages, and solved elegantly in Haskell Oh, hey, that's pretty nice... ... and a solution to a problem that you souldn't have in the first place. I mean, if you want to construct XML or SQL statements, you ought to use an abstract data type that ensures proper nesting etc. and not a simple string. Regards, apfelmus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to translate Haskell to other languages?
On Sat, 2008-10-11 at 16:55 +0100, Matthew Naylor wrote: Hi Jason, I don't know Python, but let me share some thoughts that you might find useful. First, a few questions about your manual translations. Are your functions curried? For example, can I partially apply zipWith? Also, you put a thunk around things like cons(...) --- should it not be the arguments to cons that are thunked? Now, on to an automatic translation. As you may know already, Haskell programs can be transformed to combinator programs which are quite simple and easy to work with. Here is what I mean by a combinator program: p ::= d*(a program is a list of combinator definitions) d ::= c v* = e (combinator definition) e ::= e e (application) | v (variable/argument) | c (constant: integer literal, combinator name, etc.) As an example of a combinator program, here is one that reverses the list [0,1,2]. rev v acc = v acc (rev2 acc) rev2 acc x xs = rev xs (cons x acc) cons x xs n c = c x xs nil n c = n main = rev (cons 0 (cons 1 (cons 2 nil))) nil This program does not type-check in Haskell! But Python, being dynamically typed, doesn't suffer from this problem. :-) A translation scheme, D[], from a combinator definition to a Python definition might look as follows. D[c v* = e] = def c() : return (lambda v1: ... lambda vn: E[e]) E[e0 e1] = E[e0] (E[e1]) E[v] = v E[c] = c() Here is the result of (manually) applying D to the list-reversing program. def nil() : return (lambda n: lambda c: n) def cons() : return (lambda x: lambda xs: lambda n: lambda c: c(x)(xs)) def rev2() : return (lambda acc: lambda x: lambda xs: rev()(xs)(cons()(x)(acc))) def rev() : return (lambda v: lambda acc: v(acc)(rev2()(acc))) def main() : return (rev() (cons()(0)( cons()(1)( cons()(2)( nil()(nil())) The result of main() is a partially-applied function, which python won't display. But using the helper def list(f) : return (f([])(lambda x: lambda xs: [x] + list(xs))) we can see the result of main(): list(main()) [2, 1, 0] Of course, Python is a strict language, so we have lost Haskell's non-strictness during the translation. However, there exists a transformation which, no matter how a combinator program is evaluated (strictly, non-strictly, or lazily), the result will be just as if it had been evaluated non-strictly. Let's call it N, for Non-strict or call-by-Name. N[e0 e1] = N[e0] (\x. N[e1]) N[v] = v (\x. x) N[f] = f I've cheekily introduced lambdas on the RHS here --- they are not valid combinator expressions! But since Python supports lambdas, this is not a big worry. NOTE 1: We can't remove the lambdas above by introducing combinators because the arguments to the combinator would be evaluated and that would defeat the purpose of the transformation! NOTE 2: i could be replaced with anything above --- it is never actually inspected. For the sake of interest, there is also a dual transformation which gives a program that enforces strict evaluation, no matter how it is evaluated. Let's call it S for Strict. S[e0 e1]= \k. S[e0] (\f. S[e1] (\x. k (f x))) S[v]= \k. k v S[f]= \k. k f I believe this is commonly referred to as the CPS (continuation-passing style) transformation. This is indeed a CPS transform. Specifically, a call-by-value CPS transform. There is also a call-by-name one. N[e0 e1] = \k. N[e0] (\f. f N[e1] k) N[v] = v N[c] = \k. k c ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on the JVM
Are there, or have there been Haskell ports to the JVM? Are any of them alive and well? See the thread started 9 September on Haskell and Java: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44252 Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Interesting new user perspective
On 11 Oct 2008, at 6:34 pm, apfelmus wrote: Andrew Coppin wrote: Martin DeMello wrote: http://blog.moertel.com/articles/2006/10/18/a-type-based-solution- to-the-strings-problem is a brilliant example of a common workaday problem found in other languages, and solved elegantly in Haskell Oh, hey, that's pretty nice... ... and a solution to a problem that you souldn't have in the first place. I mean, if you want to construct XML or SQL statements, you ought to use an abstract data type that ensures proper nesting etc. and not a simple string. Do you have an example of what you mean? Personally, I use stored procedures with a database as they protect from sql injection attacks (unless you write some really stupid procedures). Iain ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constraints at construction
On Oct 11, 2008, at 2:54 PM, Bit Connor wrote: Smart constructors are nice but if your module doesn't also export the regular constructor then you can't pattern match against it. It would be cool if there was a way to export constructors from a module, but only for use in pattern matching and not constructing new values. I suppose you could make a new data type to pattern match against. For example, say you have data Foo A B = hidden constructors foo :: A - B - Foo A B -- smart constructor You could introduce and export data FooView = Foo A B | other possible deconstructions fooView :: Foo A B - FooView A B You could even redefine the smart constructor as foo : FooView A B - Foo A B ... if you wanted. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Interesting new user perspective
On 11 Oct 2008, at 9:02 pm, Svein Ove Aas wrote: On Sat, Oct 11, 2008 at 9:30 PM, Iain Barnett [EMAIL PROTECTED] wrote: Personally, I use stored procedures with a database as they protect from sql injection attacks (unless you write some really stupid procedures). Isn't this what parametrized queries are for? they will also work (at least in MS SQL Server), but you'd lose some of the performance and organisational benefits. And it stops people littering code with badly written SQL statements - at least I can keep track of the procedures! :) Iain ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constraints at construction
On Thu, Oct 9, 2008 at 3:05 PM, Mitchell, Neil [EMAIL PROTECTED] wrote: Hi Iain, The wiki page has quite a nice article: http://www.haskell.org/haskellwiki/Smart_constructors Smart constructors are nice but if your module doesn't also export the regular constructor then you can't pattern match against it. It would be cool if there was a way to export constructors from a module, but only for use in pattern matching and not constructing new values. Peace, Bit ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Interesting new user perspective
On Sat, Oct 11, 2008 at 9:30 PM, Iain Barnett [EMAIL PROTECTED] wrote: Personally, I use stored procedures with a database as they protect from sql injection attacks (unless you write some really stupid procedures). Isn't this what parametrized queries are for? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to translate Haskell to other languages?
On Sat, Oct 11, 2008 at 8:55 AM, Matthew Naylor [EMAIL PROTECTED] wrote: Hi Jason, I don't know Python, but let me share some thoughts that you might find useful. First, a few questions about your manual translations. Are your functions curried? For example, can I partially apply zipWith? Also, you put a thunk around things like cons(...) --- should it not be the arguments to cons that are thunked? I don't recall if I mentioned this in my original email. My goal is to do automatic translations. So, no you can't partially apply zipWith, but then that's because Python doesn't support partial application. On the other hand, you can easily use a lambda to get around this. So in an automatic translation I would replace partial application with lambdas. This shouldn't be a problem right? My rule was to put a thunk around any Haskell value. So I put cons cells in thunks and I even wrapped functions in thunks. The exception was that there were quite a few places where I could tell by inspection that a particular value would already be in a thunk. For example, since I require in my translation that putting a value in a cons requires the value to be a thunk then when I pull values out of a cons I already know they are thunks so no need to rewrap them. Now, on to an automatic translation. As you may know already, Haskell programs can be transformed to combinator programs which are quite simple and easy to work with. Here is what I mean by a combinator program: p ::= d*(a program is a list of combinator definitions) d ::= c v* = e (combinator definition) e ::= e e (application) | v (variable/argument) | c (constant: integer literal, combinator name, etc.) As an example of a combinator program, here is one that reverses the list [0,1,2]. rev v acc = v acc (rev2 acc) rev2 acc x xs = rev xs (cons x acc) cons x xs n c = c x xs nil n c = n main = rev (cons 0 (cons 1 (cons 2 nil))) nil This program does not type-check in Haskell! But Python, being dynamically typed, doesn't suffer from this problem. :-) I plan to exploit this in my translations as well. I will assume type checked Haskell programs as input to the translator. A translation scheme, D[], from a combinator definition to a Python definition might look as follows. D[c v* = e] = def c() : return (lambda v1: ... lambda vn: E[e]) E[e0 e1] = E[e0] (E[e1]) E[v] = v E[c] = c() Here is the result of (manually) applying D to the list-reversing program. If nil() corresponds to [] in Haskell, then how did you arrive at this definition? As Derek Elkins points out your transformation is a CPS based. So I'm going to guess that c is the continuation and n represents the nil? def nil() : return (lambda n: lambda c: n) This one makes a little bit of sense to me. I see the components of the list, the x and xs, and you apply the continuation to them. What's going on with n? def cons() : return (lambda x: lambda xs: lambda n: lambda c: c(x)(xs)) Now, now this is a getting a bit hard to read :) def rev2() : return (lambda acc: lambda x: lambda xs: rev()(xs)(cons()(x)(acc))) def rev() : return (lambda v: lambda acc: v(acc)(rev2()(acc))) I'm glad I don't have to maintain code that looks like this :) def main() : return (rev() (cons()(0)( cons()(1)( cons()(2)( nil()(nil())) The result of main() is a partially-applied function, which python won't display. But using the helper def list(f) : return (f([])(lambda x: lambda xs: [x] + list(xs))) we can see the result of main(): list(main()) [2, 1, 0] Cool! So, supposing I went with a translation scheme like what you gave. I think I would end up with deeply nested function calls, this is probably very bad for the python run-time. Also, how do I allow Python to then access the Haskell values? I guess your definition of list above is an example of that, but I'm not sure how I'd pull that off in general. Of course, Python is a strict language, so we have lost Haskell's non-strictness during the translation. However, there exists a transformation which, no matter how a combinator program is evaluated (strictly, non-strictly, or lazily), the result will be just as if it had been evaluated non-strictly. Let's call it N, for Non-strict or call-by-Name. Interesting. N[e0 e1] = N[e0] (\x. N[e1]) N[v] = v (\x. x) N[f] = f I've cheekily introduced lambdas on the RHS here --- they are not valid combinator expressions! But since Python supports lambdas, this is not a big worry. Right, not so bad. My translation was doing the same thing actually. A common thing to see in my code is, x = thunk(lambda: y). NOTE 1: We can't remove the
Re: [Haskell-cafe] Re: Interesting new user perspective
Svein Ove Aas wrote: On Sat, Oct 11, 2008 at 9:30 PM, Iain Barnett [EMAIL PROTECTED] wrote: Personally, I use stored procedures with a database as they protect from sql injection attacks (unless you write some really stupid procedures). Isn't this what parametrized queries are for? Yes. (And it also improves DB performance since it doesn't have to continually reparse the query and rebuild the query plan.) Now consider dynamically constructing HTML and avoiding HTML injection attacks. There isn't an easy machine fix for that one. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Interesting new user perspective
apfelmus wrote: ... and a solution to a problem that you souldn't have in the first place. I mean, if you want to construct XML or SQL statements, you ought to use an abstract data type that ensures proper nesting etc. and not a simple string. Right. And if you have 25 KB of HTML data, you're *really* going to transform all of that into an abstract data type just to avoid injection problems, right? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to translate Haskell to other languages?
2008/10/11 Jason Dagit [EMAIL PROTECTED]:
On Sat, Oct 11, 2008 at 8:55 AM, Matthew Naylor
[EMAIL PROTECTED] wrote:
Here is the result of (manually) applying D to the list-reversing program.
If nil() corresponds to [] in Haskell, then how did you arrive at this
definition? As Derek Elkins points out your transformation is a CPS based.
So I'm going to guess that c is the continuation and n represents the nil?
def nil() : return (lambda n: lambda c: n)
I think this is known as the Church encoding. The parameters n and c
describe what to do with lists that are constructed with [] and (:),
respectively.
You can do this in Haskell, as well:
newtype List a = List { unList :: forall b. b - (a - List a - b) - b }
nil :: List a
nil = List (\n c - n)
cons :: a - List a - List a
cons x xs = List (\n c - c x xs)
foldListR :: (a - b - b) - b - List a - b
foldListR f z l = unList l z (\x xs - f x (foldListR f z xs))
compare foldListR with foldr:
foldr f z [] = z
foldr f z (x:xs) = f x (foldr f z xs)
Essentially, it represents the data in terms of how you pattern match
on it. You can in principle pull this off for any Haskell type, but
the resulting code isn't anything you'd want to work on manually.
--
Dave Menendez [EMAIL PROTECTED]
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
Thanks to everyone for their feedback. I've made some updates and
posted the PDF to my blog:
http://blog.codeslower.com/2008/10/The-Haskell-Cheatsheet
On Sat, Oct 11, 2008 at 4:33 AM, Thomas Hartman [EMAIL PROTECTED] wrote:
Very nice!
I have my own cheat list, which are haskell commands I find useful but
find inconvenient or difficult to look up in the supplied
documentation. I actually hardwire my cheats into .bashrc doing
something like
thartman_haskell_cheatting() {
cat EOF
blah blah
cheat
}
so i can quickly see all my haskell cheats using tab completion. but a
pdf is even nicer :)
**
[EMAIL PROTECTED]:~/Desktopthartman_haskell_oneliners
ghc -e '1+2'
[EMAIL PROTECTED]:~/Desktopthartman_haskell_regex_hints
cabal install pcre-regex
Most likely want:
Prelude Text.Regex.PCRE user123 =~ ^(user)(\d*)$ ::
(String,String,String,[String])
(,user123,,[user,123])
That is: (before match, match, after match, subgroups)
or maybe
Prelude Text.Regex.PCRE user123 user456 =~ (u(se)r)(\d*) ::
[[String]]
[[user123,user,se,123],[user456,user,se,456]]
if you need all submatches of all matches
since
Prelude Text.Regex.PCRE user123 user456 =~ (user)(\d*) ::
(String,String,String,[String])
(,user123, user456,[user,123])
doesn't quite do what I want -- no submatches
and there's no instance for :: (String,String,String,[String])
I don't need all submatches of all matches very often though.
:: Bool -- did it match
:: String -- first match
:: [String] -- every match
:: :: (String,String,String) -- before, matched, after
http://www.serpentine.com/blog/2007/02/27/a-haskell-regular-expression-tutorial/
[EMAIL PROTECTED]:~/Desktopthartman_haskell_testing_things
import Data.Test.HUnit
runTestTT $ TestCase $ assertEqual meh 1 2
runTestTT $ TestList [ TestCase $ assertEqual meh 1 2 ]
[EMAIL PROTECTED]:~/Desktopthartman_haskell_hints
offline documentation:
ghc-pkg describe bytestring | grep -i doc
or probably just
haddock-interfaces:
/usr/local/share/doc/ghc/libraries/bytestring/bytestring.haddock
haddock-html: /usr/local/share/doc/ghc/libraries/bytestring
note to self:
start using cabal install --global (or whatever the flag is)
so all documentation is browsable from one place
Use language pragmas, with commas
And you can't put LANGUAGE and OPTIONS_GHC in the same pragma
{-# LANGUAGE NoMonomorphismRestriction, PatternSignatures #-}
{-# OPTIONS -fglasgow-exts #-}
Debugging
toVal.hs:30:17:
Couldn't match expected type 'blee'
against inferred type 'bleh'
bleh is whatever is at 30:17
blee is something that's wanted by whatever is calling the value at 30:17
If the error is in the definition of some function,
then probably one function case conflicts with another, you can
ignore other functions.
In this case you will only get one line:col to look at.
If there are more than one line:col to look at, possibly separate
functions are in conflict.
So, smart to always fix in the definition of type errors first.
Still baffled? Won't compile?
Give top-level functions type signatures. Won't hurt, might help.
:set -fwarn-missing-signatures
or {-# OPTIONS -fwarn-missing-signatures #-}
Start commenting out calling functions until it compiles, and then
look at the signatures.
And then type the signatures in explicitly... does something look funny?
Like, wrong number of args? Maybe currying went wrong.
tag and bundle a distribution:
darcs tag 0.2
cabal configure
cabal sdist
cd dist; unzip, verify install does the right thing
http://hackage.haskell.org/packages/upload.html
check upload, and upload.
see also http://en.wikibooks.org/wiki/Haskell/Packaging
group module imports from multiple modules in one place:
module MyInductiveGraph (
module Data.Graph.Inductive,
module EnoughFlow
)
where
import Data.Graph.Inductive
import EnoughFlow
**
2008/10/11 Justin Bailey [EMAIL PROTECTED]:
All,
I've created a cheat sheet for Haskell. It's a PDF that tries to
summarize Haskell 98's syntax, keywords and other language elements.
It's currently available on hackage[1]. Once downloaded, unpack the
archive and you'll see the PDF. A literate source file is also
included.
If you install with cabal install cheatsheet, run cheatsheet
afterwards and the program will tell you where the PDF is located.
The audience for this document is beginning to intermediate Haskell
programmers. I found it difficult to look up some of the less-used
syntax and other language stumbling blocks as I learned Haskell over
the last few years, so I hope this document can help others in the
future.
This is a beta release (which is why I've limited the audience by
using hackage) to get feedback before
Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0
On Sat, Oct 11, 2008 at 5:30 AM, Holger Siegel [EMAIL PROTECTED] wrote: - The explanation of the layout rule is wrong. If you define more than one value in a let declaration, then it is only required that the identifiers start on the same column. Thank you - updated. - When I started to learn Haskell, I had problems with the use of (.) and ($). I had learned what function application and lambda abstractions look like, but then I looked at Haskell code written by experienced Haskellers and found expressions like (map (succ . succ) $ 1:xs) that I did not understand. A small section describing how to read such expressions could be useful for beginners. Me too. I had a section on that originally but cut it due to time. Patches are always welcome :) - the section about do-notation is more a mini-tutorial than a cheatsheet. Instead, you could show two or three examples that demonstrate how do- notation, list comprehensions and the operator = relate. That is what I had to look up more than once until I got used to it. There is also an example of what is wrong, where showing the right thing would have sufficed. True, but I think it's helpful. That stuff really confused me at first. Justin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] OT: Haskell desktop wallpaper?
Magnus Therning wrote: Where can I find some nice wallpapers inspired by Haskell, or maybe even created by Haskell code? I once suggested a futuristic feature for xmonad or xmonad plugin: screensaver that randomly picks a haskell lecture video to play or a haskell tutorial page to display. The idea is that your colleagues do not know haskell yet, but they pass by your screen and learn haskell. Perhaps it can be extended to wallpaper as well. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Congrats. to wxHaskell
I just want to congratulate the wxHaskell team on their new release. With GHC 6.10 RC, cabal 1.6, and cabal-install 0.6, I was able to simply, cabal install wxcore wx And it all worked. Well done! -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [] vs [()]
Sam Danielson wrote: The [] constructor takes no arguments and is like Nothing in the Maybe type. The list : (cons) infix constructor takes two arguments, an element of type a and a list of type a, to construct a new list. Compare to Maybe. data []a = [] | a : [a] data Maybe a = Nothing | Just a Another way of saying [()] is ():[] which, comparing with the Maybe type, is similar to saying Just () but Just only takes one argument where (:) takes two. Both List and Maybe are containers that have a null constructor, namely [] and Nothing. ():[] contains () similar to how Just () contains (). You can make your own list type and put () in it as follows. Or, in Monad terms: [()] and Just () are both return () in their respective Monads. [] and Nothing are both mzero in their respective MonadsPluses. (Both are also fail in their respective Monads, but I find fail's presence in Monad a bit inelegant, though handy.) -- src/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Ubuntu Haskell
Don Stewart wrote: * Arch now has 609 Haskell packages in AUR. Have you thought about doing this for Ubuntu? If you know how to automatically generate packages, you could set up a PPA (private package archive) on Launchpad. -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Ubuntu Haskell
ashley: Don Stewart wrote: * Arch now has 609 Haskell packages in AUR. Have you thought about doing this for Ubuntu? If you know how to automatically generate packages, you could set up a PPA (private package archive) on Launchpad. I've spoken with Jeremy Shaw, who has similar systems in place, based also on Cabal (and cabal-install), for generating native packages for Debian Ubuntu. However, we do need some Ubuntu champions who can work incrementally, over a long time, to keep packages up to date on their systems. Automation makes this a *lot* easier, so you may only need one or two people, motivated to work on their distro. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shooting your self in the foot with Haskell
John Van Enk wrote: On Sun, Oct 5, 2008 at 11:21 PM, John Van Enk [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: On Sun, Oct 5, 2008 at 6:22 PM, Simon Richard Clarkstone [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: BTW, these could go on the wiki. I'll see about putting them there. :) http://haskell.org/haskellwiki/Shooting_your_self_in_the_foot This needs to be cleaned up a little (lots of dups, though they are all great). You missed my one, though I think the ballistics algebra one is better. ISTR that the point of the original list was originally to show that, though you can screw up with C, with every other language you can screw up in far more complicated and inscrutable ways. Ballistics algebra is indeed a way to screw up that is not possible in C. (Darnit Thunderbird, why don't you DWIM when I hit reply or reply-all?) -- src/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: How to translate Haskell to other languages?
Jason Dagit [EMAIL PROTECTED] writes: I don't recall if I mentioned this in my original email. My goal is to do automatic translations. So, no you can't partially apply zipWith, but then that's because Python doesn't support partial application. On the other hand, you can easily use a lambda to get around this. So in an automatic translation I would replace partial application with lambdas. This shouldn't be a problem right? Partial application can be done in python, though somewhat awkwardly. Take a look at functools.partial. -- Green's Law of Debate: Anything is possible if you don't know what you're talking about. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
