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Today's Topics:
1. Re: IO Problem (Brent Yorgey)
2. Re: Mathematical Blundering (Andrew Sackville-West)
3. requested module name differs from name found in interface
file (Larry Evans)
4. A Pascal-like Language Compiler (Pranesh Srinivasan)
5. Re: A Pascal-like Language Compiler ( Joel Bj?rnson )
6. Re: requested module name differs from name found in
interface file (Larry Evans)
----------------------------------------------------------------------
Message: 1
Date: Fri, 17 Oct 2008 14:11:55 -0400
From: Brent Yorgey <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] IO Problem
To: beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset=us-ascii
On Fri, Oct 17, 2008 at 07:58:45PM +1300, Jamie McCloskey wrote:
>
> All this creates a whole load of functions with IO, even though it
> should not be necessary. What I would like to know, is how I can avoid
> this, possibly by modifying exec to just take a State.
The short answer is: it IS necessary. Think about what exec does, for
instance. It takes a Program and an initial State and does... what?
Executes the program, which might result in some things getting
printed to the screen or some input being read from the keyboard.
That is, exec can have some I/O effects. Therefore, exec's return
type must be an IO value. The same goes for all the other functions
you mentioned.
This is a feature, not a bug --- the type of a function tells you
precisely whether it can possibly have any I/O effects. If the return
type is (IO something), then it can; if the return type does not
involve IO, then it is *guaranteed*[1] that it cannot have any I/O
effects.
-Brent
[1] Except for that pesky unsafePerformIO. But you should never use
that, unless you really absolutely know what you are doing and why.
------------------------------
Message: 2
Date: Fri, 17 Oct 2008 16:52:28 -0700
From: Andrew Sackville-West <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] Mathematical Blundering
To: beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="us-ascii"
On Thu, Oct 16, 2008 at 09:57:39AM +0100, Paul Johnston wrote:
> Bit of basic maths.
> You are using a power series to approximate sine
> This works by taking an expansion about a fixed point, usually zero.
> It only works well around that point.
> If you get far away it works badly.
> You need to exploit the cyclic nature of the trignometrical functions i.e.
> Sin x = sin ((2 * pi) + x) = sin ((4 * pi) + x)
> Essentially consider the shift in multiples of 2 * pi and calculate the value
> of x nearest to zero.
>
> See
> http://en.wikipedia.org/wiki/Taylor_series
> The diagram on the top right is very instructive.
how appropriate. We're doing this in math right now ;)
ISTM that OP should take x modulo pi and (per the wiki diagram) a
seven term series to get a nice usable sine function. (and don't
forget to watch for the sign (nopun) of the result)
fact 1 = 1
fact x = x * (fact $ x - 1)
term n x = (-1)**n * (theta**(2*n + 1) / fact(2*n + 1))
where theta = pi * ((x / pi) - xTrunc) * sign
xTrunc = fromIntegral $ truncate (x/pi)
sign = (-1)**xTrunc
sine x = sum $ take 7 [term n x | n<-[0..]]
Wow, that took me a lot longer to figure out because of all the mixing
of numeric types. The critical part was fromIntegral in the poorly
name xTrunc function.
but it looks pretty accurate.
*Main> maximum [sine x - sin x | x<-[1..100]]
1.2652798913048713e-5
and it's easy to boost the accuracy by taking more terms in the
series.
taking 9:
*Main> maximum [sine x - sin x | x<-[1..100]]
1.1683279538265978e-8
taking 11:
*Main> maximum [sine x - sin x | x<-[1..100]]
4.694897248747054e-12
I'm sure there must be a better way to get x modulo pi, but I don't
know enough...
I'd love some feedback on my solution both from a math and a haskell
perspective.
A
>
> Paul
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jeffrey Drake
> Sent: Thursday, October 16, 2008 9:47 AM
> To: Haskell Beginners
> Subject: [Haskell-beginners] Mathematical Blundering
>
>
> I have defined myself a set of functions to test:
>
> fact 1 = 1
> fact n = n * (fact $ n - 1)
>
> sine x = x - (x^3/(fact 3)) + (x^5/(fact 5)) - (x^7/(fact 7))
>
> Where my code is 'sine' and the prelude's is sin:
> *Main> sine 1
> 0.841468253968254
> *Main> sin 1
> 0.8414709848078965
>
> *Main> sine 2
> 0.9079365079365079
> *Main> sin 2
> 0.9092974268256817
>
> *Main> sine 3
> 9.107142857142847e-2
> *Main> sin 3
> 0.1411200080598672
>
> *Main> sine 4
> -1.3841269841269837
> *Main> sin 4
> -0.7568024953079282
>
> After 2 they seem to diverge rather rapidly, and I am not sure why. Any ideas?
>
> I would have thought that 4 terms would have been enough.
>
> - Jeff.
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
> _______________________________________________
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> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 3
Date: Sun, 19 Oct 2008 16:59:39 -0500
From: Larry Evans <[EMAIL PROTECTED]>
Subject: [Haskell-beginners] requested module name differs from name
found in interface file
To: beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="iso-8859-1"
With a file containing:
> module Main where
>
> import Array
> import Control.Functor.Fix
I get:
> make
> ghc -i/root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 -c
> catamorphism.example.hs
>
> catamorphism.example.hs:19:0:
> Bad interface file:
> /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2/Control/Functor/Fix.hi
> Something is amiss; requested module main:Control.Functor.Fix
> differs from name found in the interface file
> category-extras-0.53.5:Control.Functor.Fix
> make: *** [all] Error 1
I used cabal to install category-extras:
> /home/evansl/download/haskell/libs # cabal install category-extras
> Resolving dependencies...
> Downloading category-extras-0.53.5...
> Configuring category-extras-0.53.5...
> Preprocessing library category-extras-0.53.5...
...
> /usr/bin/ar: creating dist/build/libHScategory-extras-0.53.5.a
> Installing library in /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2
> Registering category-extras-0.53.5...
> Reading package info from "dist/installed-pkg-config" ... done.
> Saving old package config file... done.
> Writing new package config file... done.
> /home/evansl/download/haskell/libs #
What should I do to import the Functor.Fix as shown here:
http://comonad.com/reader/2008/recursion-schemes/
TIA.
-Larry
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Message: 4
Date: Mon, 20 Oct 2008 17:39:50 +0530
From: "Pranesh Srinivasan" <[EMAIL PROTECTED]>
Subject: [Haskell-beginners] A Pascal-like Language Compiler
To: "Hasekll - Beginners" <beginners@haskell.org>
Message-ID:
<[EMAIL PROTECTED]>
Content-Type: text/plain; charset=ISO-8859-1
Hi all,
I have been learning haskell for the last month or so. I have just
started becoming familiar with monads and looking at Parsec.
As part of the Language Translators Course this semester, in the
university I study, we are to implement a Pascal-like toy language to C
translator[1]. The aim is to take a simple subset of Pascal and
translate it to an intermediate 3 operand format, before translating it
to simple C.
The time I have for this project is about a month. I am contemplating
doing this in Haskell, since I have read in several places that Haskell
(and other functional languages) is very good for writing compilers. In
fact, I have started writing code for lexing the basic tokens.
I do have doubts about my ability to finish it. The curiosity to do
something of a moderate size in Haskell, and the extremely friendly
folks on #haskell, tipped me on this decision.
I looked around for similar things on the internet, in vain. I have
several doubts at this stage some of which include,
* how the parser will determine the scope of the variables,
i.e, how can a symbol table be implemented (in the state of the
parser ?)
* how easy or difficult will basic code optimisation (like unused
variables, and loops with no side effects) and type checking be?
* how would one go about implementing basic error recovery in the
parser (make it skip the current line, or block) ?
I would love it if I get a few pointers from people who have been there,
done that.
Or am I being too ambitious too early? :)
[1] One of the suggested programming exercises at the end of the
"Dragon" Compilers book, by Aho, et al.
--
Pranesh S
------------------------------
Message: 5
Date: Mon, 20 Oct 2008 15:47:44 +0200
From: " Joel Bj?rnson " <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] A Pascal-like Language Compiler
To: "Pranesh Srinivasan" <[EMAIL PROTECTED]>
Cc: Hasekll - Beginners <beginners@haskell.org>
Message-ID:
<[EMAIL PROTECTED]>
Content-Type: text/plain; charset=ISO-8859-1
Oops, should have hit the replay-to-all button, so sorry Pranesh for
double posting.
Hi,
have you considered using BNCF[1]? With BNFC you can define a grammar
for your language then let the BNFC converter generate a Haskell
parser (along with an ADT representing syntax trees).
On Mon, Oct 20, 2008 at 2:09 PM, Pranesh Srinivasan <[EMAIL PROTECTED]> wrote:
> I looked around for similar things on the internet, in vain. I have
> several doubts at this stage some of which include,
>
> * how the parser will determine the scope of the variables,
> i.e, how can a symbol table be implemented (in the state of the
> parser ?)
I think it is probably better to exclude the variable look up, type
checking etc from the parsing phase. Here's an example of a general
scheme:
1. Parse input file into an abstract syntax tree representation.
2. Perform type checking on your syntax tree.
3. Transform the syntax tree using re-write rules and optimisations.
4. Pretty print the syntax tree in order to output code of your target language.
>
> * how easy or difficult will basic code optimisation (like unused
> variables, and loops with no side effects) and type checking be?
I would say probably easier then in many other languages since Haskell
has such a great support for recursive data types, pattern matching
etc.
> * how would one go about implementing basic error recovery in the
> parser (make it skip the current line, or block) ?
>
Not really sure what you mean by error recovery but when using BNFC
all of the tedious work of implementing the parsers is automated.
Regards,
Joel
[1] http://www.cs.chalmers.se/Cs/Research/Language-technology/BNFC/
------------------------------
Message: 6
Date: Mon, 20 Oct 2008 12:15:39 -0500
From: Larry Evans <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] requested module name differs from
name found in interface file
To: Justin Bailey <[EMAIL PROTECTED]>, beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="iso-8859-1"
On 10/20/08 11:38, Justin Bailey wrote:
> On Mon, Oct 20, 2008 at 9:25 AM, Larry Evans <[EMAIL PROTECTED]> wrote:
>
[snip]
>
>> Do you mean install it manually as shown in the aforementioned
>> How_to_install_a_Cabal_package?
>>
>
> No, I mean go to http://hackage.haskell.org and download the .tar.gz
> file directly
> (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras).
> Unzip/untar that package. I haven't looked at it, but there is
> probably a "src" directory. Put your example file in there and load it
> in ghci. Your source file should then use the source available in that
> directory tree rather than the category-extras install which seems
> broken. This works because the GHC will look at the import
> "Control.Functor.Fix" and use that to look for
> "Control/Functor/Fix.hs" from the current directory.
>
Tried that but:
<--- cut here ---
GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
Prelude> :load
"/home/evansl/download/haskell/libs/category-extras-0.53.5/src/catamorphism.example.hs"
Control/Category/Dual.hs:17:7:
Could not find module `Control.Category':
Use -v to see a list of the files searched for.
Failed, modules loaded: none.
Prelude>
<--- cut here ---
If it's any help, the directory list for src/Control/Category is:
<--- cut here ---
/home/evansl/download/haskell/libs/category-extras-0.53.5/src/Control/Category:
total used in directory 52 available 148798604
drwxr-xr-x 3 evansl evansl 4096 Jun 30 08:08 .
drwxr-xr-x 9 evansl evansl 4096 Jun 30 08:08 ..
-rw-r--r-- 1 evansl evansl 2118 Jun 30 08:08 Associative.hs
-rw-r--r-- 1 evansl evansl 1668 Jun 30 08:08 Braided.hs
drwxr-xr-x 2 evansl evansl 4096 Jun 30 08:08 Cartesian
-rw-r--r-- 1 evansl evansl 4973 Jun 30 08:08 Cartesian.hs
-rw-r--r-- 1 evansl evansl 1020 Jun 30 08:08 Discrete.hs
-rw-r--r-- 1 evansl evansl 1340 Jun 30 08:08 Distributive.hs
-rw-r--r-- 1 evansl evansl 668 Jun 30 08:08 Dual.hs
-rw-r--r-- 1 evansl evansl 578 Jun 30 08:08 Hask.hs
-rw-r--r-- 1 evansl evansl 3254 Jun 30 08:08 Monoidal.hs
-rw-r--r-- 1 evansl evansl 1746 Jun 30 08:08 Object.hs
>--- cut here ---
> You might have more luck finding out what's going on by emailing
> Haskell-Cafe directly, too.
>
Yes, thanks.
> Justin
>
>
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