Tom Schrijvers wrote:
data EvalDict sem = EvalDict { val :: Int - sem Int, add :: sem Int
- sem Int - sem Int }
An alternative option is to capture the structure in a GADT:
data Eval a where
Val :: Int - Eval Int
Add :: Eval Int - Eval Int - Eval Int
And then write what were instances
On 3/31/10 12:44, Heinrich Apfelmus wrote:
go Next (Single x t1) = liftM (Single x) (rewrite f t1)
go Next (Fork t1 t2 ) = liftM2 Fork (rewrite f t1)
(rewrite f t2)
In particular, liftM and liftM2 make it apparent that we're
+1 for lifting this restriction.
On 4/4/10 23:28, David House wrote:
An issue came up on #haskell recently with Hackage accounts requiring
real names.
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Dear café (CC John and Chris),
I'm having some trouble with getting positional parameters in HDBC-mysql
to work. Most of the time they work fine, but sometimes (and often
enough that it's a serious bother) the parameters don't reach the server
correctly.
Let me first describe my setup:
*
On 4/6/10 15:31, Heinrich Apfelmus wrote:
In fact, it doesn't actually work for monads, I think you have to wrap
it in a newtype. :D The same effect can be achieved with `ap` , though:
Fortunately, by now most (standard) monads are also applicatives. :-)
Besides generalizing to an arbitrary
Thanks! That's great news. Yes, all seems fine now. :-)
It was a very interesting bug to isolate. At one point I was in the
situation where compiling with -O2 fixed the problem and -O0 didn't,
seemingly consistently.
By the way, I got two warnings while compiling:
* Warning: Fields of
On 5/3/10 23:46, Jason Dagit wrote:
This happened to a co-worker on her mac. We used gdb to track the bus
errors to the network library. Once we tracked it down to there, we did
some combination of deleting $HOME/.cabal, building/installing the
latest version of Network and then relinking
On 6/14/10 10:39, Ivan Lazar Miljenovic wrote:
By being told that using them would solve some problem you're
complaining about on #haskell or the mailing lists, you look at
examples, read up on them, etc.
Short version: don't worry about advanced concepts until you have to.
If all else fails,
On 6/14/10 0:10, Roman Cheplyaka wrote:
Of course most parsers don't consume trailing newlines. But I was
writing general function to use in many places in the code which would
recover the end location. In most cases it just subtracts 1 from the
column number, but what if it just happened so
On 7/8/10 21:36, Stephen Tetley wrote:
Hello
I suspect you will have to choose single examples for each of the
patterns/ abstractions you are interested in.
Doaitse Swierstra's library UU.Parsing is the originator or the
Applicative style. Its latest incarnation is the library
uu-parsinglib.
On 7/21/10 12:48, José Romildo Malaquias wrote:
Hello.
I have noticed that I do not receive duplicate copies of messages from
haskell-cafe, although Avoid duplicate copies of messages? is set to
No in the mailing list membership configuration.
I want the copies because I archive all the
On 7/28/10 14:53, S. Doaitse Swierstra wrote:
see: file:///Users/doaitse/.cabal/share/doc/uu-parsinglib-2.4.2/html/index.html
Readers might have more luck with the following URLs:
http://hackage.haskell.org/package/uu-parsinglib
On 7/27/10 9:58, Sebastian Fischer wrote:
On Jul 27, 2010, at 9:15 AM, Sjoerd Visscher wrote:
Oh, by the way, with noMatch, eps, alt and seq_ RegExp is itself a
Semiring,
Yes, but it's hard to define an Eq instance for arbitrary regular
expressions that reflects equivalence of regexps.
On 7/30/10 9:29, Stefan Holdermans wrote:
Jason,
There is one case where you can break out of a monad without knowing which
monad it is. Well, kind of. It's cheating in a way because it does force the
use of the Identity monad. Even if it's cheating, it's still very clever and
On 7/30/10 12:29, Tillmann Rendel wrote:
C K Kashyap wrote:
I am of the
understanding that once you into a monad, you cant get out of it?
That's not correct.
There are many monads, including Maybe, [], IO, ... All of these monads
provide operations (=), return and fail, and do notation
Dear café,
Given:
instance Category C
y :: forall r. C r (A - r)
I am looking for the types of x and z such that:
x . y :: forall r. C r r
y . z :: forall r. C r r
Can you help me find such types? I suspect only one of them exists.
Less importantly, at least to me at this moment: how do I
Bas van Dijk wrote:
Comments and patches are highly welcome.
I tried to install on my Mac but bindings-common choked on:
cabal install bindings-common
Resolving dependencies...
cabal: Error: some packages failed to install:
bindings-common-1.1 failed while unpacking the package. The
Roel van Dijk wrote:
Yes, that happens. I don't now the cause but the work-around is easy.
Simply download the package manually from hackage, unpack and install
using cabal.
At least the following packages suffer from this problem:
bindings-common
bindings-libusb
bindings-posix
Perhaps
minh thu wrote:
Also, I'd like to know why
id id True
is permitted but not
(\f - f f True) id
Because this requires rank-2 types:
Prelude :set -XScopedTypeVariables
Prelude :set -XRank2Types
Prelude (\(f :: forall a. a - a) - f f True) id
True
HTH,
Martijn.
Daryoush Mehrtash wrote:
I am trying to learn more about concurrent applications in Haskell by
studying an existing a real application source code. I would very much
appreciate if you can recommend an application that you feel has done a
good job in implementing a real time application in
Hallo café,
Can anyone think of datatypes that are Foldable but not Traversable?
If not, what is the purpose of having a separate Foldable class?
Thanks,
Martijn.
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Ross Paterson wrote:
On Fri, Oct 09, 2009 at 04:41:05PM +0200, Martijn van Steenbergen wrote:
Can anyone think of datatypes that are Foldable but not Traversable?
Set
Nice! Thank you all for your answers.
Martijn.
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Felipe Lessa wrote:
which unfortunately needs {-# LANGUAGE RecursiveDo #-} or
some ugliness from mfix
But mdo/mfix is awesome! :-(
Martijn.
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Brandon S. Allbery KF8NH wrote:
My fix would be to have myPrefixOf require the prefix be terminated in
whatever way is appropriate (end of input, white space, operator?)
instead of simply accepting as soon as it gets a prefix match regardless
of what follows.
Maybe you can use notFollowedBy
Erik de Castro Lopo wrote:
However after reading the hackage descriptions of both Transformers and
MTL, it seems that they share a very similar heritage. I therefore hacked
the iteratee.cabal file and replaced the build-depends on transformers
with one on mtl and the package built quite happily.
Dear café,
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ImpredicativeTypes #-}
type Void = forall a. a
newtype Mono a = Mono { runMono :: [Void] }
beep :: Mono a - Mono a
beep (Mono vs) = Mono (map undefined vs)
Compiling this with GHC results in:
Monotype.hs:9:28:
Cannot match a
Hello café,
I've never written an Alternative instance for a newtype yet that
doesn't look like this:
instance Alternative T where
empty = T empty
T x | T y = T (x | y)
Why does newtype deriving not work for Alternative? (It works fine for
Monoid.)
Thanks,
Martijn.
You guys are right. I was being silly. Thanks. :-)
Ryan Ingram wrote:
Works for me on GHC6.10.4:
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It doesn't work for this one:
newtype Split a = Split { runSplit :: [Either a (Char, Split a) ]}
But my handwritten instance remains identical.
Groetjes,
Martijn.
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David Virebayre wrote:
Taking the opportunity to thank very much both Simons and Ian for the
work they do and the enthusiasm they show. You guys rock.
I heartily second that!
Martijn.
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Peter Verswyvelen wrote:
For learning, I would like to develop my own implementation of type
inference, based on the paper Typing Haskell in Haskell.
At first sight, the source code of THIH contains a small number of
tests, but I was wandering if a large test set exist?
I'm pretty sure GHC
Bonjour café,
data ExprF r
= Add r r
| Sub r r
| Mul r r
| Div r r
| Num Int
This is a well-known pattern that for example allows nice notation of
morphisms. But what is it called? I've heard fixed-point view, open
datatypes and some others, but I'm curious where
Waldemar Biernacki wrote:
Is there the eval function like in imperative languages?
You mean like in interpreted languages?
I'd like to write an application which has to be compiled to exec file. It is
neccessary to performe some additional procedures which are unknown at the
moment of the
Anton van Straaten wrote:
On the plus side, this does make for a slogan with high market appeal:
Haskell: Kittens inside
Thanks. Now I have trouble getting this image of lambda-shaped bonsai
kittens out of my head.
;-)
Martijn.
___
Thanks for all the pointers, guys. You've been very helpful.
I also found Type-indexed data types (Hinze et al) to be a good source.
Much appreciated!
Martijn.
Martijn van Steenbergen wrote:
data ExprF r
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Niklas Broberg wrote:
Actually, it seems something went awry. I got a 500 Internal Server
Error on my cabal upload, the package is there on hackage but it seems
it was never added to the list of packages. This means cabal update
doesn't know about it, nor is it listed on the What's New page.
Hello!
I'm happy to announce the first release of HoleyMonoid, a datatype that
helps you build monoids with holes in them. The holes are filled in
later using normal function application. For example:
let holey = now x =
. later show
. now , y =
.
Magnus Therning wrote:
IIRC James Gosling once said that if he were to design Java today he would
leave out classes. I suppose partly due to many of the issues with data
inheritance.
This sounds interesting. Can you link us to an article, please?
Thanks,
Martijn.
Yusaku Hashimoto wrote:
Hello cafe,
Do you know any data-type which is Applicative but not Monad?
The Except datatype defined in the Applicative paper.
Some parsers are not monads, allowing for optimizations.
Martijn.
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Hi Ozgur,
Ozgur Akgun wrote:
pKeyword_Int = ( \ _ _ _ - int ) $ pSym 'i' * pSym 'n' * pSym 't'
pKeyword_Float = ( \ _ _ _ _ _ - float ) $ pSym 'f' * pSym 'l'
* pSym 'o' * pSym 'a' * pSym 't'
As you can see there is an obvious pattern if you try to capture a
keyword. If there were a function
Dear café,
I am looking for a function that does an N-dimensional diagonal
traversal. I want the traversal to be fair: the sum of the indices of
the produced combinations should be non-decreasing. Let me illustrate
with an example.
The type of a 2-dimensional traversal would look like this:
Louis Wasserman wrote:
+1 on Control.Monad.Omega. In point of fact, your diagN function is simply
diagN = runOmega . mapM Omega
You'll find it an interesting exercise to grok the source of
Control.Monad.Omega, obviously, but essentially, you're replacing
concatMap with a fair (diagonal)
Henning Thielemann wrote:
On Wed, 4 Nov 2009, Sjoerd Visscher wrote:
On Nov 4, 2009, at 3:21 PM, Twan van Laarhoven wrote:
I looked on hackage but I was surprised that I couldn't find
this simple
monad. The package level-monad does look very similar, only it
uses a
Tony Morris wrote:
I have two projects that I intend to put on hackage soon. One depends
on the other. I have cabaled both. I am wondering how others work
with this kind of set up where changes are made to both libraries as
they work.
You just update and re-upload the packages as necessary. It
Hello,
Are there currently any known problems that would hinder the
implementation of kind polymorphism [1], e.g. unresolved inelegancies or
technical limitations, or is it only a matter of finding the time to
implement it?
Thanks,
Martijn.
[1]
Simon Peyton-Jones wrote:
It used to be, because GHC used to implement so-called deep skolemisation.
See Section 4.6.2 of
http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-rank/putting.pdf
Deep skolemisation was an unfortunate casualty of the push to add impredicative
Hello,
I have a type family and a type class:
type family ErrorAlg (f :: (* - *) - * - *) e ix :: *
class MkErrorAlg f where
mkErrorAlg :: ErrorAlg f e a - f (K0 a) ix - Either e a
Instances for these two really go hand in hand, so I thought I would
move the type family into the type
Sebastian Fischer wrote:
Does anyone know why types and values are in separate namespaces but
classes and types are not?
Good question. I don't know the answer, but it is interesting to note
that the report explicitly mentions this decision (but provides no reason):
An identifier must not
zaxis wrote:
then how to use `X` ? Would you mind explaining the newtype X in detail ?
You can just think of a newtype as a normal datatype:
data X a = X (ReaderT XConf (StateT XState IO) a)
I.e. construction and pattern matching work indentically.
Every newtype you will find will have
Hi Sean,
Sean Leather wrote:
Perhaps this isn't answering your question, but you can turn the above
into an associated type as follows.
class MkErrorAlg f where
type ErrorAlg (f :: (* - *) - * - *) :: * - * - *
mkErrorAlg :: ErrorAlg f e a - f (K0 a) ix - Either e a
That is
So here's a totally wild idea Sjoerd and I came up with.
What if newtypes were unwrapped implicitly?
What advantages and disadvantages would it have?
In what cases would this lead to ambiguous code?
Thanks,
Martijn.
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Don Stewart wrote:
vandijk.roel:
On Wed, Dec 2, 2009 at 11:44 PM, Gregory Crosswhite
gcr...@phys.washington.edu wrote:
On a more serious note, Download Haskell /= Download Haskell Platform, so if I were glancing
down the sidebar looking for a link to download the Haskell Platform then the
Luke Palmer wrote:
On Fri, Dec 4, 2009 at 10:26 AM, Radek Micek radek.mi...@gmail.com wrote:
Hello.
I have two types for expression:
data Expr = Add Expr Expr | Mul Expr Expr | Const Int
data AExpr = AAdd AExpr AExpr | AConst Int
The first one supports addition and multiplication and the
Hi Radek,
Radek Micek wrote:
I can write a function to simplify the first expression:
simplify :: Expr - Expr
simplify = {- replaces:
a*1 and 1*a by a,
a+0 and 0+a by a -}
And I would like to use the function simplify for the second type
AExpr. What can I do is to convert AExpr to Expr,
Radek Micek wrote:
Hi,
thank you for your reply but your MulExpr
does not support expressions like
(2*3)+5
Oh! You're right, how silly of me.
Martijn.
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Eugene Kirpichov wrote:
Hello.
Consider the type: (forall a . a) - String.
On one hand, it is rank-2 polymorphic, because it abstracts over a
rank-1 polymorphic type.
On the other hand, it is monomorphic because it isn't actually
quantified itself: in my intuitive view, a parametrically
Max Cantor wrote:
That stuffed me up for a bit. I wrote some ugly template haskell a while back
to automatically generate XmlPickler instances. can send to you if you want
I recall typLAB writing about generic XML picklers:
http://blog.typlab.com/2009/11/writing-a-generic-xml-pickler/
Hello,
Above error is one of those that appear when using GHC on the 64-bit
Snow Leopard. I've already patched my /usr/bin/ghc to mention -optc-m32
-opta-m32 -optl-m32. However, the error still occurs when doing 'cabal
haddock' in *some* packages.
For example, a local project of mine builds
Gregory Collins wrote:
Martijn van Steenbergen mart...@van.steenbergen.nl writes:
Hello,
Above error is one of those that appear when using GHC on the 64-bit Snow
Leopard. I've already patched my /usr/bin/ghc to mention -optc-m32 -opta-m32
-optl-m32. However, the error still occurs when doing
Günther Schmidt wrote:
Hi,
when I cabal-installed the iteratee package, the transformers package
was also installed as a dependency.
Now when I run applications that import Control.Monad.Transformers I get
this:
Could not find module `Control.Monad.Trans':
it was found in
Niklas Broberg wrote:
Haskell '98 apparently features 25 reserved words. (Not counting forall
and mdo and so on, which AFAIK are not in Haskell '98.)
21 actually. case, class, data, default, deriving, do, else, if,
import, in, infix, infixl, infixr, instance, let, module, newtype, of,
then,
Dear café,
I am deeply impressed with Vacuum[1][2], Ubigraph[3] and especially
their combination[4]. I can trivially and beautifully visualize the ASTs
that my parser produces. I can visualize zippers of the ASTs and confirm
that sharing is optimal.
Ubigraph is also able to animate graph
Antoine Latter wrote:
getResponse = do
require 256
x - getX
len - getWord16be
y - getY
z - getZ
require (fromIntegral len * 8)
a - getA
b - getB
return $ Response x y z a b c
This looks like code that could be written in applicative style, in
which case you could analyze the
Edward Kmett wrote:
Functional references let you both read and write 'attributes' in a
structure. These can be chained to access members of members.
You can also use them to build bidirectional views on fields (and
compose those again as well).
Martijn.
Ryan Ingram wrote:
Unfortunately, this makes things like
infinite_xs - sequence (repeat arbitrary)
no longer work, since the state never comes out the other side.
You're asking to execute an infinite number of monadic actions. How can
this ever terminate at all?
Martijn.
Ivan Miljenovic wrote:
On 16 February 2010 08:35, Don Stewart d...@galois.com wrote:
Enjoy the new decade of flexible, fusible, fast arrays for Haskell!
/me points out that 2010 is actually the last year of the decade, and
not the first year of a new decade...
There certainly is /a/ decade
Are you saying that GHC complains about an unused import that is in fact
used? Perhaps you've run into this bug:
http://hackage.haskell.org/trac/ghc/ticket/1148
Are you using a recent version of GHC?
Groetjes,
Martijn.
On 8/10/10 22:22, Lyndon Maydwell wrote:
Hi Cafe.
I have written some
On 8/2/10 7:09, Ertugrul Soeylemez wrote:
Given the definition of a Haskell function, Haskell is a pure language.
The notion of a function in other languages is not:
int randomNumber();
The result of this function is an integer. You can't replace the
function call by its result without
On 8/10/10 23:27, Felipe Lessa wrote:
If we had in C:
return (randomNumber(10, 15) + randomNumber(10, 15))
That would not be the same as:
int x = randomNumber(10, 15)
return (x + x)
That's not fair. You're comparing C's '=' with Haskell's '='. But you
should be comparing C's '='
On 8/10/10 23:53, Felipe Lessa wrote:
and the result is IO Int. When we replace the function call by its
result, I think it is fair to replace the C function call by an int
and the Haskell function call by an IO Int, because that is what
those functions return.
Fair enough. :-)
Also, a
I have just booked 5 beds in Hostel 47 in Ghent for our group. We will
be staying in a 6-bed room, so there is still one bed available. If
anyone is still looking for a bed in Ghent and wants to share a room
with fellow Haskellers, that one bed might be an interesting choice. I
told them we
Hi Mark, Chris,
I had trouble as well but just found this pretty recent blog post by @freels
which worked like a charm for me:
http://matt.freels.name/2010/hdbc-mysql-os-x.html
I hope it works for you as well!
I've added Chris to the list of recipients because maybe he can incorporate
the fix
Hello cafe,
When I want to locally install my own package through cabal install it
tries to reinstall dependencies convertible-1.0.9.1, HDBC-2.2.6.1 and
HDBC-mysql-0.6.3 even though they are already installed (and work fine).
Why does it do this?
cabal-install version 0.8.2
using version
Dear cafe,
Following the instructions at
http://matt.freels.name/2010/hdbc-mysql-os-x.html I successfully
installed HDBC-mysql on my Mac!
However when doing a cabal install in a local package which depends on
HDBC-mysql, cabal proceeds to reinstall HDBC-mysql (is this a bug? see
my other
Thomas Davie wrote:
I've found user installs don't work at all on OS X, various people in
#haskell were rather surprised to discover this, so apparently it's not
the default behavior on other platforms.
It really rather makes cabal install rather odd – because it doesn't
actually install
Hi Uwe,
Uwe Hollerbach wrote:
I have so far only tested it with ghc 6.8.3 on MacOS 10.3.9 (powerPC),
but I know of no reason why it wouldn't work with other versions and
OSs.
It works fine on 6.10.1 on Leopard Intel as well.
I'm afraid I haven't messed with cabal much yet, so it's not
David Leimbach wrote:
Just refuse to use UHC until it conforms. One can refuse to use GHC
libraries that use extensions as well for similar reasons. I always
think twice when I see something that isn't Haskell 98 in my stack.
Do you not use Hugs for the same reason?
Jeff Heard wrote:
First of all, it's crashed only once, and the error was an actual
segfault, so I'm not sure what went wrong there. All in all, I like
the eyecandy and have left it on for everything I do, but I've noticed
that arrows have an extra space after them, no matter which arrow.
Also,
Matti Niemenmaa wrote:
In order to run properly, list-tries needs a version of 'containers'
that hasn't yet been released. I incorporated a little hack which makes
it compile even with 0.2, but some calls will fail by calling 'error':
30 of my 1014 test cases do so.
1014 test cases?! Wow. :-)
Dear Haskell Hackers,
Many thanks to all those who attended Hac5! We had a spectacular number
of participants: over 50 hackers showed up, representing several
countries. Many thanks also to the organising committee and sponsors!
Those bagels on Sunday were delicious. :-)
Please check out
Hoi Peter,
Peter Verswyvelen wrote:
Sure I understand what a GADT is, but I'm looking for practical
examples, and the ones on the wiki seem to show what you *cannot* do
with them...
I use GADTs for two things:
1) Type witnesses for families of data types. An example from the
MultiRec
Hi Denis,
Denis Bueno wrote:
where the rCompare field would be a function that is based on the
flags passed to the command-line problem. But this has an ugly
asymmetry. Does anyone have any other ideas?
Here's a solution that is more symmetrical but not necessarily prettier:
newtype Wrap =
Denis Bueno wrote:
The problem here is that the order is fixed. Statically. I can't
change it at runtime based on flags. (Right? Unless I'm missing
something)
That is right. It might or might not be a problem in your specific case.
sortBy :: (a - a - Ord) - [a] - IO [a]
sortBy cmp =
Tillmann Rendel wrote:
Achim Schneider wrote:
In other words:
1) Explain Pointed
2) Explain Functor
3) Explain Applicative
4) Explain Monad
Why Pointed first? Functor seems more useful and more basic.
They are in order of power: every monad is an applicative; every
applicative is a
siki wrote:
I'm not sure if this is possible at all. I'd like to do something like this:
class A a where
foo :: a - Double
foo a = 5.0
class (A a) = B a where
foo a = 7.0
This is currently not possible in Haskell. It's been proposed, though:
Steffen Schuldenzucker wrote:
Uhm, isn't it:
class (Functor f) = Pointed f where
pure :: a - f a -- singleton, return, unit etc.
Got it from: The Typeclassopedia by Brent Yorgey (forgot the URL, sorry)
Yes, but also: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/54685
So maybe
Hi Steve,
Steve wrote:
Why is gcd 0 0 undefined?
That's a good question. Can you submit an official proposal?
http://www.haskell.org/haskellwiki/Library_submissions
Thanks,
Martijn.
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Hi Nicolas,
Nicolas Martyanoff wrote:
So now I'd want to use it for a small project of mine, a simple
multiplayer roguelike based on telnet. I wrote a minimal server in C, and
it took me a few hours. Now I'm thinking about doing the same in Haskell,
and I'm in trouble.
I don't know if this is
Magnus Therning wrote:
Without the `seq` the call to sleep will simply be skipped (is there an
easier way to force evaluation there?). Without `unsafePerformIO` all
the sleeping is done up front, and all numbers are print at once at the
end.
The goal is of course to use code along the same
Hi Deniz,
Deniz Dogan wrote:
So, basically I'd like some sort of folding functionality for these
data types, without having to hack the lexer/parser myself
(parameterising the data types), because as I said they're being
generated by BNFC.
What exactly do you mean by folding functionality?
Hello,
Mr. McBride and mr. Paterson define in their Applicative paper:
data Except e a = OK a | Failed e
instance Monoid e = Applicative (Except e) where ...
Sometimes I'd still like to use = on Excepts but this feels wrong
somehow, because it doesn't use monoids nicely like the Applicative
Andrew Wagner wrote:
[quote]
Here's [a]language to to interpret (where postfix * means tupling):
Variables: x
Integer literals: i
Terms:
t = Lambda x*. t
| Apply t t*
| Var(x)
| Num(i)
Can someone explain to me how I should read this? It supposedly explains
what the postfix
mwin...@brocku.ca wrote:
Hi,
I am using parsec to parse a small programming language. The language is typed
and
I need to do some type checking, too. I have decided to do the parsing and type
checking
simultaneously in the my parsec parser. This approach avoids to keep source
code positions
Rouan van Dalen wrote:
Hi everyone.
I am designing my own programming language.
I would like to know what is the best way to go about writing my compiler in
haskell.
What are the tools available in haskell that can help with compiler
construction?
I know about Happy. Is that a good tool to
por...@porg.es wrote:
-- or with Data.Function.Predicate (shameless plug)
http://hackage.haskell.org/packages/archive/predicates/0.1/doc/html/Data-Function-Predicate.html:
Whoa, a function called isn't, why is this the first time I see that? :-)
Martijn.
Bonjour café,
A small puzzle:
Consider the function inTwain that splits a list of even length evenly
into two sublists:
inTwain Hello world!
(Hello ,world!)
Is it possible to implement inTwain such that the recursion is done by
one of the standard list folds?
Is there a general way to
Hello,
Suppose I have two projects: 1) one that defines a monad transformer and
an accompanying type class that captures my monad-specific operations
and 2) one that uses the other project, combining the monad transformer
with, say, Parsec.
Now while writing my Parsec parser I want to use
Hi John,
John Ky wrote:
full = do
let myOrder = init -- [1]
{ item = Just init
{ itemId = Something
}
, operation = Just Buy
}
putStrLn $ show myOrder
return ()
Where initOrder and initItem
Thomas ten Cate wrote:
Possible, yes.
Efficient, not really.
inTwain = foldr (\x (ls, rs) - if length ls == length rs then (x:ls, rs) else
(x:(init ls), (last ls):rs)) ([], [])
But this uses length and init and last all of which are recursive
functions. I consider that cheating: only
Geoffrey Marchant wrote:
The linked paper appears to show the right style.
This appears to satisfy the conditions, however:
inTwain as = let (x,y,_) = foldr (\a (r,s,t) - case (t) of {b:(b':bs)
- (r,a:s,bs); _ - (a:r,s,t)}) ([],[],as) as in (x,y)
This one is very interesting. Thanks. :-) It
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