Re: [Haskell-cafe] Type question re: map
map :: (a - b) - [a] - [b] map takes a function and transforms a list of a's to b's. map succ [1,2,3] == [succ 1, succ 2, succ 3] == [2, 3, 4] In general, map f :: [a] - [b] where a is domain-type of f and b is image-type of f (f :: a - b). map map [x, y, z] == [map x, map y, map z] so, x,y,z should functions of type (a - b). and the result list, [map x, map y, map z], should have type [([a] - [b])] because map x :: [a] - [b]where x :: a - b On 3/6/09, R J rj248...@hotmail.com wrote: Given the following (usual) definition of map: map:: (a - b) - [a] - [b] map f [] = [] map f (x : xs) = f x : map f xs What's the type of map map? GHCi's :t command reveals: *Main :t map map map map :: [a - b] - [[a] - [b]] I'd be grateful if anyone could provide a systematic type calculation so that I can reason through more complicated examples myself. Thanks. _ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] C-like Haskell
Did you print it? I'm using same code with ghc --make -O2 and it takes forever to finish. On Wed, Jan 28, 2009 at 8:06 PM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: On Wed, 2009-01-28 at 16:42 -0800, drblanco wrote: I do already have the number I wanted, but was wondering how this could be made faster, or even why it's so slow. This is all on GHC 6.8.3 under OS X Intel, using ghc -O2. I'm not exactly sure what's different, but for me it works pretty well. I put back in the Int64 type signature. For comparison, the C code below runs in 1 second. You've got a faster machine than me :-) I compiled both the Haskell and C versions to standalone executables with ghc/gcc -O2 and ran them with time. C version: $ time ./circ 3141592649589764829 real0m2.430s user0m2.428s sys 0m0.000s Haskell version: time ./circ2 3141592653589764829 real0m2.753s user0m2.756s sys 0m0.000s Not too bad I'd say! :-) I was using ghc-6.10 for this test. It would appear that ghc-6.8 is a bit slower, I get: 3141592653589764829 real0m5.767s user0m5.768s sys 0m0.000s Now the other difference is that I'm using a 64bit machine so perhaps ghc just produces terrible code for Int64 on 32bit machines. Duncan ___ 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] Simplification of this function?
saml @pl myFunc f (a,b) (c,d) = (f a c, f b d)
lambdabot myFunc = (`ap` snd) . (. fst) . flip flip snd . ((flip . (ap .))
.) . flip flip fst . ((flip . ((.) .)) .) . (flip = (((.) . flip . (((.) .
(,)) .)) .))
why not use zipWith?
[a,b] `g` [c,d] where g = zipWith f
2009/1/16 Andrew Wagner wagner.and...@gmail.com
I've been playing around with this, but haven't been able to come up with
anything.
myFunc f (a,b) (c,d) = (f a c, f b d)
It feels as if there should be a nice simple version of this using some
combination of {un,}curry, on, , ***, or something else.
Any thoughts?
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Re: [Haskell-cafe] Simplification of this function?
Sorry it won't work because list should be homogeneous.
I think your myFunc is standard.
This guy names it thread:
http://alaska-kamtchatka.blogspot.com/2009/01/essence-of-concatenative-languages.html
On Fri, Jan 16, 2009 at 10:32 AM, sam lee skyn...@gmail.com wrote:
saml @pl myFunc f (a,b) (c,d) = (f a c, f b d)
lambdabot myFunc = (`ap` snd) . (. fst) . flip flip snd . ((flip . (ap
.)) .) . flip flip fst . ((flip . ((.) .)) .) . (flip = (((.) . flip .
(((.) . (,)) .)) .))
why not use zipWith?
[a,b] `g` [c,d] where g = zipWith f
2009/1/16 Andrew Wagner wagner.and...@gmail.com
I've been playing around with this, but haven't been able to come up with
anything.
myFunc f (a,b) (c,d) = (f a c, f b d)
It feels as if there should be a nice simple version of this using some
combination of {un,}curry, on, , ***, or something else.
Any thoughts?
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Re: [Haskell-cafe] Time for a new logo?
http://i35.tinypic.com/mjon83.png used this: http://www.simwebsol.com/ImageTool/Default.aspx 2008/12/14 George Pollard por...@porg.es: ? ___ 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] Parsec and (then?) type-check
I use my type checking monad, which is separate from Parsec's monad. So, I can't think of a way to type check during parsing in Parsec's monad. Anyways, this is what I did: data Expr = ... | At SourcePos Expr SourcePos is from Parsec. Basically, my parse actions will return (At pos e). And I pass At pos e to typeCheck action. typeCheck (At pos e) = do put pos -- typeCheck is in State monad. -- in case of error, I'll pos - get and report source position. typeCheck e typeCheck (Variable a) = do ... check out this: http://www.lipl.googlepages.com/index.html#source On Fri, Dec 12, 2008 at 5:06 PM, Greg Fitzgerald gari...@gmail.com wrote: Parser gurus, When you write a parser with a library like Parsec, do you typically type-check while parsing, or afterward in a separate pass? The latter is more modular, but it means labeling every element in the AST with the parser position so that you can give good error messages. Do you find the added modularity worth the hassle or just pack type-checking into the parser pass? Thanks, Greg ___ 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] SDL program under MinGW
Yes, with some modifications. I used SDL-1.2.13, not SDL-1.2.12. Also, my Include-Dirs has include\SDL, not include: C:\Users\client\code\SDL-1.2.13\include\SDL If I don't put include\SDL, I get errors like Graphics\UI\SDL\General.hsc:1:17: SDL.h: No such file or directory And I used the following command to configure: C:\Users\client\Downloads\SDL-0.5.4runghc Setup.lhs configure --prefix=C:\Users\client\haskell --datadir=$prefix\share --user it builds and installs fine. I only get linker error (undefined reference to SDL_main) when I compile a program that uses SDL binding. On Sun, Dec 7, 2008 at 4:23 AM, Bit Connor [EMAIL PROTECTED] wrote: Did you follow the instructions described in the WIN32 file? On Sat, Dec 6, 2008 at 2:05 PM, sam lee [EMAIL PROTECTED] wrote: Hi. I am using Windows Vista. I installed the following: - ghc 6.8.3 (using official Windows binary installer) - MinGW (from http://nuwen.net/mingw.html) - SDL binding (from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) I am trying to compile the following program: module Main where import qualified Graphics.UI.SDL as SDL main = do SDL.init [SDL.InitEverything] SDL.quit I get the following error: C:\Users\client\code\sandboxghc --make Example.hs Linking Example.exe ... C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: undefined reference to `SDL_main' According to SDL faq (http://www.libsdl.org/faq.php?action=listentriescategory=4#48) I should use int main(int argc, char *argv[]), not int main() nor WinMain(). Can anyone compile the above snippet on windows machine? I am using SDL library that is shipped with MinGW distribution that I'm using. I also recompiled SDL haskell binding with official SDL windows binary (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I get the same undefined reference to SDL_main linker error. Do I need to have specific main IO action so that SDL would work in Haskell? Thank you. Sam. ___ 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] SDL program under MinGW
Hi. I am using Windows Vista. I installed the following: - ghc 6.8.3 (using official Windows binary installer) - MinGW (from http://nuwen.net/mingw.html) - SDL binding (from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) I am trying to compile the following program: module Main where import qualified Graphics.UI.SDL as SDL main = do SDL.init [SDL.InitEverything] SDL.quit I get the following error: C:\Users\client\code\sandboxghc --make Example.hs Linking Example.exe ... C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: undefined reference to `SDL_main' According to SDL faq (http://www.libsdl.org/faq.php?action=listentriescategory=4#48) I should use int main(int argc, char *argv[]), not int main() nor WinMain(). Can anyone compile the above snippet on windows machine? I am using SDL library that is shipped with MinGW distribution that I'm using. I also recompiled SDL haskell binding with official SDL windows binary (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I get the same undefined reference to SDL_main linker error. Do I need to have specific main IO action so that SDL would work in Haskell? Thank you. Sam. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] SDL program under MinGW
I can compile http://darcs.haskell.org/~lemmih/hsSDL/hssdl/Examples/MacOSX/Test.hs But I can't find the small Cabal file and c_main.c . Does anyone have those files? I want to try if Test.hs links well on windows platform. I think the binding needs to not include SDLMain and instead do the initialization itself; see the rest of that FAQ question. I can't grep for SDLMain in the binding. How should I modify the binding code to use WinMain + proper initialization? Thanks. Sam. On Sat, Dec 6, 2008 at 5:19 PM, Ryan Ingram [EMAIL PROTECTED] wrote: From the link you provided (http://www.libsdl.org/faq.php?action=listentriescategory=4#48) Q:I get Undefined reference to 'SDL_main' ... A:Make sure that you are declaring main() as: #include SDL.h int main(int argc, char *argv[]) In particular notice that they want you to include SDL.h. I suspect they are doing something scary like #define main(ac, av) SDL_Main(ac, av, maybe some other args) The problem is that the library itself is defining WinMain, so you've got a fight between the Haskell compiler (which wants to define the entry point for your program to start up the runtime), and the SDL library (which is trying to be helpful and failing). I think the binding needs to not include SDLMain and instead do the initialization itself; see the rest of that FAQ question. In fact, the MACOSX readme file in the Haskell SDL binding talks about exactly this problem. -- ryan On Sat, Dec 6, 2008 at 2:05 PM, sam lee [EMAIL PROTECTED] wrote: Hi. I am using Windows Vista. I installed the following: - ghc 6.8.3 (using official Windows binary installer) - MinGW (from http://nuwen.net/mingw.html) - SDL binding (from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) I am trying to compile the following program: module Main where import qualified Graphics.UI.SDL as SDL main = do SDL.init [SDL.InitEverything] SDL.quit I get the following error: C:\Users\client\code\sandboxghc --make Example.hs Linking Example.exe ... C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: undefined reference to `SDL_main' According to SDL faq (http://www.libsdl.org/faq.php?action=listentriescategory=4#48) I should use int main(int argc, char *argv[]), not int main() nor WinMain(). Can anyone compile the above snippet on windows machine? I am using SDL library that is shipped with MinGW distribution that I'm using. I also recompiled SDL haskell binding with official SDL windows binary (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I get the same undefined reference to SDL_main linker error. Do I need to have specific main IO action so that SDL would work in Haskell? Thank you. Sam. ___ 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] bug in time libs?
I can't find document for System.Time .
But I can import System.Time . It's weird...
I can't find document for TimeDiff and related functions. I guess this
is all deprecated.
A related bug report: http://hackage.haskell.org/trac/ghc/ticket/2519
I would use Data.Time.Clock
Prelude :m + Data.Time.Clock
Prelude Data.Time.Clock getCurrentTime
Prelude Data.Time.Clock t0 - getCurrentTime
Prelude Data.Time.Clock t1 - getCurrentTime
Prelude Data.Time.Clock t0
Prelude Data.Time.Clock diffUTCTime t0 t1
-2.912s
Prelude Data.Time.Clock diffUTCTime t1 t0
2.912s
Prelude Data.Time.Clock :t diffUTCTime t1 t0
diffUTCTime t1 t0 :: NominalDiffTime
On Mon, Dec 1, 2008 at 10:05 PM, Jason Dusek [EMAIL PROTECTED] wrote:
I'm on 6.10.1, using the libs provided with the binary of GHC
for Mac OS X. I was fooling around with the time package in
GHCi and something seems to be off. I have included a
transcript.
If I take a time diff, then the seconds are positive when the
picos are negative and vice versa.
--
_jsn
|...transcript...|
Prelude System.Time :m + System.Time
Prelude System.Time t0 - System.Time.getClockTime
Prelude System.Time t1 - System.Time.getClockTime
Prelude System.Time diffClockTimes t0 t1
TimeDiff { tdYear = 0
, tdMonth = 0
, tdDay = 0
, tdHour = 0
, tdMin = 0
, tdSec = -7
, tdPicosec = 23884600
}
Prelude System.Time diffClockTimes t1 t0
TimeDiff { tdYear = 0
, tdMonth = 0
, tdDay = 0
, tdHour = 0
, tdMin = 0
, tdSec = 7
, tdPicosec = -23884600
}
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Re: [Haskell-cafe] '#' in literate haskell
# is significant because it can be sh-bang line or pre-processor.
The only way I can think of is:
alias lhspp=sed 's/^#//'
ghc --make -F -pgmF lhspp File.lhs
On Sat, Nov 29, 2008 at 10:07 PM, John MacFarlane [EMAIL PROTECTED] wrote:
Can anyone explain why ghc does not treat the following
as a valid literate haskell program?
- test.lhs
# This is a test
foo = reverse . words
When I try to load this in ghci (or compile it using ghc),
I get:
test.lhs:1:2: lexical error at character 'T'
It seems that the problem is the '#' character in the first
column. Replacing it with something else, or moving it to
the right one space, solves the problem.
The following literate haskell program, from
http://notvincenz.blogspot.com/2008/01/literate-haskell-and-c.html,
also fails to load for me, for the same reason (the leading
'#' in line 8).
literate-haskell-and-c.lhs ---
/* c and lhs file
module Foo where
main = print Haskell
*/
#include
int main() {
printf(C\n);
return 0;
}
I've reproduced this with ghc 6.10.1 and ghc 6.8.3 (linux binaries
from haskell.org) and with ghc 6.8.2 (Ubuntu intrepid).
Interestingly, hugs (September 2006 version) has no trouble with
test.lhs. I haven't tried ghc 6.6.
I care about this because I'd like to use markdown conventions
to format the comment parts of literate haskell programs. Markdown
supports atx-style headers, which begin with strings of '#'
characters starting in the first column. I know that some people use
markdown with literate haskell, so there must be something basic here
that I'm missing!
John
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Re: [Haskell-cafe] SOEGraphics in GHC?
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HGL or http://hackage.haskell.org/cgi-bin/hackage-scripts/package/soegtk 2008/11/24 Dmitri O.Kondratiev [EMAIL PROTECTED]: Please help, to locate in GHC distribution SOEGraphics library from Paul Hudak, book The Haskell School of Expression (http://www.haskell.org/soe/ ) Thanks! ___ 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] What *not* to use Haskell for
I haven't found multitrack audio recording applications written in Haskell. These are usually written in C++ using Jack audio or ASIO. This probably means that it is not a good idea to write real time audio applications in Haskell at the moment. So, probably avoid writing applications that use a high frequency timer and IO that should be synchronous to the timer in Haskell. On Tue, Nov 11, 2008 at 9:02 PM, wren ng thornton [EMAIL PROTECTED] wrote: Dave Tapley wrote: Hi everyone So I should clarify I'm not a troll and do see the Haskell light. But one thing I can never answer when preaching to others is what does Haskell not do well? Usually I'll avoid then question and explain that it is a 'complete' language and we do have more than enough libraries to make it useful and productive. But I'd be keen to know if people have any anecdotes, ideally ones which can subsequently be twisted into an argument for Haskell ;) With the appropriate caveats about particular subdomains (see final paragraph), I wouldn't use Haskell for scripting. That is, (1) for Bash-style programming where 95% of the code is just invoking *nix jobs, or (2) for very simple yet regex-heavy scripts where Perl/Awk/Sed is often used. Re #1: Honestly, I don't see anything other than a dedicated competitor being able to unseat Bourne/Bash at this task. Certainly a competitor would have much room for improvement-- what with being able to replace string-rewriting semantics with term-rewriting semantics, vastly improving type safety and catching innumerable bugs. However, with unsavory frequency, it is exactly those type-unsafe substitutions which can make shell scripting cleaner and more direct than a type-safe alternative. Having type safety as well as this sort of non-compositional structure would take a good deal of work to get right. Re #2: People often complain about spooky Perl that uses things like implicit $_ or other hidden variables. While these constructs can make any sizable project unmaintainable, for the quick and dirty jobs they're just what's needed to get things done with clarity. While ByteString code using regexes is just as fast in Haskell, it's often more than twice as long as the Perl, Sed, or Awk equivalents because many of the basic control structures (like Perl's -n, -p, -l,... flags) aren't already provided. That said, this isn't necessarily a bad thing for Haskell. Real programming languages often don't do so well in these areas (Perl being the exception), and they don't feel too bad about it. Both varieties of shell scripting are very much of the DSL nature; for programs with a significant amount of actual logic instead of mere plumbing or regexing, Haskell can certainly outshine these competitors. On the one hand, C and friends fight dirty and much work has been done so Haskell can join in on the bit-bashing glory. However, shell scripting is a whole different kind of imperative muck and very little work (that I've seen) has tried to get Haskell to jump down in the sewers with them. -- Live well, ~wren ___ 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] An interesting curiosity
I think it's because + is left associative. and pattern matching is
top to bottom.
1 + 1 + 1 = (1 + 1) + 1 = found match: 1 + 1 = 3 = 3 + 1 = found
match: 3 + 1 = 3 = 3
On Thu, Sep 18, 2008 at 11:01 PM, Steven Shaw [EMAIL PROTECTED] wrote:
2008/9/19 Evan Laforge [EMAIL PROTECTED]
let {1 + 1 = 3; 3 + 1 = 3} in 1 + 1 + 1
Which gives 3.
Perhaps even more confusing is:
let {1 + 1 = 3; 3 + 1 = 3} in 3 + 1 + 1
which gives 3
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Re: [Haskell-cafe] How to do this in FP way?
I can think of 2 ways.
module Main where
import Control.Monad.State
First, normal way:
diff (now, old) = (now - old, now)
diff takes now and old and returns result (now - old) and modified old (now).
For example,
diff (diff (1,0))
== diff (1 - 0, 1)
== diff (1, 1)
== (1 - 1, 1)
== (0, 1)
I think people use the word threaded to describe what diff is doing:
the variable old is threaded through many calls to diff.
testDiff = diff . diff . diff . diff . diff . diff $ (2, 1)
testDiff returns (2,1)
Second way is using monads:
diff' now = do
old - get
put now
return (now - old)
diff' uses State monad.
If you're not familiar with monads, State monad does similar to what
diff function does (it threads the variable old).
But, being a monadic action, diff' looks like imperative version
syntactically. It gives illusion of having global variable (old).
testDiff' = do
result - diff' 2
result - diff' result
result - diff' result
result - diff' result
result - diff' result
result - diff' result
return result
runTestDiff' = runState testDiff' 1
runTestDiff' also returns (2,1)
2008/6/15 Magicloud Magiclouds [EMAIL PROTECTED]:
Hello,
I am getting familiar with FP now, and I have a program design kind of
question.
Say I have something like this in C:
static int old;
int diff (int now) { /* this would be called once a second */
int ret = now - old;
old = now;
return ret;
}
Because there is no variable in Haskell. So how to do this in a FP
way?
Thanks.
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[Haskell-cafe] M1 + M2 = M3 where both computations in M1 and M2 can be used?
Hi.
I want to compose two monads to build another monad where
computations of the two monads can be used inside.
I have:
- MonadTypeInfer : interface (class) for TypeInfer monad
- TypeInfer : a monad that has Map String Type (association of names and types)
- TypeInferT : transformer of above monad
- MonadEval : interface (class) for Eval monad
- Eval : a monad that has Map String Expr (association of names and
code/function body)
- EvalT : transformer of Eval
- tInfer :: Expr - TypeInfer Type -- given expr, returns type of it
in TypeInfer monad
- eval :: Expr - Eval Expr -- given expr, returns normalized expr in Eval monad
Problem: in repl, when user defines a function, it should type check
and register type of the function to TypeInfer monad's Map String
Type.
Also, it should store the expression of the function in Eval monad.
I build REPL monad using TypeInferT and EvalT.
newtype REPL a = REPL { runREPL :: TypeInferT (EvalT IO) a }
deriving(Monad, Functor, MonadIO, MonadTypeInfer, MonadEval)
repl :: REPL ()
repl = do
input - prompt
case parse input of
Left err - -- handle error
Right expr - do
t - tInfer expr -- BAD!! tInfer :: TypeInfer Type
println (show t)
result - eval expr -- BAD!! eval :: Eval Expr
println (show result)
repl
Should I make tInfer :: REPL Type, eval :: REPL Expr?
Is there a way to build a monad where you could use sub-monads'
(monads used to build current monad) computations?
I prefer keeping tInfer :: TypeInfer Type, eval :: Eval Expr
because tInfer never uses actions in Eval monad and vice versa.
It seems like what I am asking is to break the type system.
Maybe I should just make them run in REPL monad.
Thank you.
Sam.
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Re: [Haskell-cafe] M1 + M2 = M3 where both computations in M1 and M2 can be used?
tInfer :: MonadTypeInfer m = Expr - m Type eval :: MonadEval m = Expr - m Expr That solves! I should've left out type annotation. On Mon, May 12, 2008 at 10:38 AM, Twan van Laarhoven [EMAIL PROTECTED] wrote: sam lee wrote: Hi. I want to compose two monads to build another monad where computations of the two monads can be used inside. I have: - MonadTypeInfer : interface (class) for TypeInfer monad - TypeInfer : a monad that has Map String Type (association of names and types) - TypeInferT : transformer of above monad - MonadEval : interface (class) for Eval monad - Eval : a monad that has Map String Expr (association of names and code/function body) - EvalT : transformer of Eval - tInfer :: Expr - TypeInfer Type -- given expr, returns type of it in TypeInfer monad - eval :: Expr - Eval Expr -- given expr, returns normalized expr in Eval monad Is there a way to build a monad where you could use sub-monads' (monads used to build current monad) computations? A solution to this problem is to use type classes, and in particular MonadTrans. You can then give an instance of MonadTypeInfer for EvalT m where m is an instance of MonadTypeInfer, and similarly an instance MonadEval for TypeInferT m. How this is implemented depends on the Monads in question, but if you use the monad transformer library with newtype deriving you can just add deriving MonadTrans. class Monad m = MonadTypeInfer m where -- functions -- tiStuff :: X - m Whatever class Monad m = MonadEval m where -- functions -- instance Monad m = MonadTypeInfer (TypeInferT m) where -- implementation -- tiStuff = ... instance Monad m = MonadEval (EvalT m) where -- implementation -- instance MonadEval m = MonadTypeInfer (EvalT m) where -- lift the functions from TypeInfer through the EvalT type, -- MonadTrans from the mtl might help here tiStuff x = lift (tiStuff x) tInfer :: MonadTypeInfer m = Expr - m Type eval :: MonadEval m = Expr - m Expr Twan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
