Re: [Haskell-cafe] Rigid type variable error
On Fri, Jun 26, 2009 at 8:48 PM, Darryn djr...@aapt.net.au wrote: From: Darryn djr...@aapt.net.au To: beginn...@haskell.org Subject: Rigid type variables match error Date: Sat, 27 Jun 2009 10:18:13 +0930 Hi, I wonder if anyone can explain what is going on here and what to do about it. I'm fairly new to Haskell, so apologies in advance if my question seems naive. I've cut my code down to a minimum that reproduces the problem. I receive an error in GHCI in the following code, complaining that it cannot match the rigid type variables for the instance definition for Ainst for the function a3. Can anyone advise about what to do about it? class A a where a1 :: a a2 :: a - a a3 :: (B b) = b - a class B b where b1 :: Int - b data (B b) = Ainst b = I | J (Ainst b) | K b instance (B b) = A (Ainst b) where a1 = I -- a2 :: (B b, A a) = a - a a2 = J -- a3 :: (B b, A a) = b - a a3 = K -- Error! -- a3 = K `asTypeOf` a3 -- Error even with this! data Binst = Val Int instance B Binst where b1 = Val Test5.hs:17:9: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the instance declaration at Test5.hs:12:12 `b' is a rigid type variable bound by the type signature for `a3' at Test5.hs:5:13 Expected type: b - Ainst b1 Inferred type: b - Ainst b In the expression: K `asTypeOf` a3 In the definition of `a3': a3 = K `asTypeOf` a3 Failed, modules loaded: none. Thanks in advance for any help anyone can provide. Let me rephrase your definitions slightly: instance (B b1) = A (Ainst b1) where a1 = I -- a2 :: (B b1, A a) = a - a a2 = J -- a3 :: (B b1, A a) = b - a a3 = K -- Error! -- a3 = K `asTypeOf` a3 -- Error even with this! This is the way the type system sees your types. Notice that in the type signature of a3, it has no way to know that b1 is the same as b, and worse yet, b is totally free and b1 is not (must be an instance of B), so it thinks they cannot be unified. Which is to say, in general they may range over different types. I would recommend removing the type class constraint from the definition of Ainst. I haven't tried it, but I think that will make life easier for you. Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trivial pivoting for the DSP lu decomposition
Hi all I created the beginnings of a simple circuit simulator using haskell. It can be downloaded from. http://plan9.bell-labs.com/sources/contrib/fernan/escomma.tar.bz2 It uses a modified version of the haskell DSP library matrix. I extended it with a simple pivoting Its not very haskelly, but it works with matrices that didn't work with the unmodified version. thanks for those that answered my list processing questions. I used that code to create a permutation vector for the pivoting. I am now looking into non-linear matrix solver. does anybody have a reference code for that? fernan -- http://www.fernski.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] A Reader Monad Tutorial
Dear Group, If any of you are struggling with understanding monads, I've tried to put together a pretty through explanation of what is behind the Reader monad. If you're interested, have a look at: http://www.maztravel.com/haskell/readerMonad.html Enjoy. Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] F# mailing list?
Hi guys, is there a mailing list for haskellers that defected to F#? Not that I was I going to, just asking, absolutely hypothetically. Uhm. Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] F# mailing list?
Hi Günther, I would be interested in one too. I'm a Haskeller currently working for an F# shop. There's hubFS but I would absolutely prefer a mailing list. --A On Sat, Jun 27, 2009 at 8:31 PM, GüŸnther Schmidtgue.schm...@web.de wrote: Hi guys, is there a mailing list for haskellers that defected to F#? Not that I was I going to, just asking, absolutely hypothetically. Uhm. Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Kind Regards, Anton Tayanovskyy F# Developer IntelliFactory, Budapest - Hungary www.intellifactory.com http://www.intellifactory.com/ DISCLAIMER AND CONFIDENTIALITY CAUTION: This message and any of its attachments (message) are intended solely for the use of the addressees and contain information that is legally privileged and confidential. Any unauthorized dissemination, distribution or copying is strictly prohibited. If you received this message in error you are obliged to delete it, destroy any printed copies, and notify the sender. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: F# mailing list?
Hi Anton, cool I'll keep you in the loop. Would you mind talking about more about what you do in the F# Shop? I must admit I only came to haskell itself only 2 years ago, created 2 apps in it and have no clue for instance on how F# standing in the market is. Günther Anton Tayanovskyy schrieb: Hi Günther, I would be interested in one too. I'm a Haskeller currently working for an F# shop. There's hubFS but I would absolutely prefer a mailing list. --A On Sat, Jun 27, 2009 at 8:31 PM, GüŸnther Schmidtgue.schm...@web.de wrote: Hi guys, is there a mailing list for haskellers that defected to F#? Not that I was I going to, just asking, absolutely hypothetically. Uhm. Günther ___ 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] Re: Could FFI support pass-by-value of structs?
Maurício schrieb: It's not usual, but it is allowed to have values of structs passed between functions directly instead of using pointers: (...) Would it be possible to allow that in Haskell FFI (...) There are a couple problems with this. First, the storage layout for a given C struct may be radically different depending on the back end, When you say struct layout, do you mean the offset of diferent fields? I don't think this would be a problem, as your sugestion of hsc2hs is one of many solutions to that. I'm not sugesting that peek and poke methods of Storage instances should be created automatically, I understand this would not worth the effort. However, isn't just knowing the size and alignment enough to write a generic struct handler that, by using the appropriate calling convention, is going to work with any struct? If not, I agree with you it's really not worth it (as we can use pointers as Felipe sugested. I tried that in http://hackage.haskell.org/package/storable-record/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: Cal3D animation library
The Cal3D for Haskell project provides a partial binding to the C++ Cal3D animation library. The project homepage is http://haskell.org/haskellwiki/Cal3d_animation There are three packages available on hackage: * cal3d-0.1 is a Haskell binding to the Cal3D library itself. * cal3d-opengl-0.1 adds a few functions for using Cal3D with OpenGL. * cal3d-examples-0.1 provides a simple example based on the Cally Demo. Cal3D is a C++ library for skeletal-based character animation. It is platform-independent and independent of any particular graphics API. In fact, it does not actually render the animation, but provides hooks that the graphics API (such as OpenGL) can use to do the actual drawing. (As far as I know, though, the only graphics API available for Haskell is OpenGL.) For more information about the (C++) Cal3D library, please see * Cal3D FAQ: http://cal3d.sourceforge.net/docs/api/html/cal3dfaq.html * Cal3D Homepage: http://home.gna.org/cal3d/ -- ___ ___ __ _ / _ \ / _ \| || | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \/\_/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Network.CGI -- practical web programming example.
I am somewhat new to haskell. It is amazing that I can actually write a CGI program using Network.CGI without really being comfortable with the Haskell type system. Especially when it involves monad transformations. So I decided that I better understand this. I looked at the Practical Web Programming examples to try to understand what is going on. I came up with a problem that might demonstrate my misunderstanding. I am wondering if you can answer questions I have. Code 1 is the example from PWP, Code 2 is my variation and it works so I am stumped by what the liftM is required. code 1 #!/usr/bin/runghc import Network.CGI import Text.XHtml import qualified Data.ByteString.Lazy as BS import Control.Monad (liftM) import Data.Maybe (fromJust) uploadDir = ../upload fileForm = form ! [method post, enctype multipart/form-data] [afile file, submit Upload] saveFile n = do cont - (liftM fromJust) $ getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f cont return $ paragraph (Saved as +++ anchor ! [href f] f +++ .) page t b = header thetitle t +++ body b basename = reverse . takeWhile (`notElem` /\\) . reverse cgiMain = do mn - getInputFilename file h - maybe (return fileForm) saveFile mn output $ renderHtml $ page Upload example h main = runCGI $ handleErrors cgiMain Code 2 (modifier) import Network.CGI import Text.XHtml import qualified Data.ByteString.Lazy as BS import Control.Monad (liftM) import Data.Maybe (fromJust) uploadDir = ../upload fileForm = form ! [method post, enctype multipart/form-data] [afile file, submit Upload] saveFile n = do cont - getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f (fromJust cont) return $ paragraph (Saved as +++ anchor ! [href f] f +++ .) page t b = header thetitle t +++ body b basename = reverse . takeWhile (`notElem` /\\) . reverse cgiMain = do mn - getInputFilename file h - maybe (return fileForm) saveFile mn output $ renderHtml $ page Upload example h main = runCGI $ handleErrors cgiMain Questions === 1) Why did the author choose to insert liftM in function saveFile? It doesn't seem necessary in my version. 2) My background mainly is Java but here is my understanding of Monad Transforms. The CGIT m type carries around with it the CGI Request context and response contexts. The transformations (lifts) is similar to casting so that you can use the functions for specific manifestations but it also encapsulates the data. Is this correct? Edward ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Network.CGI -- practical web programming example.
Your code examples are: On Sat, Jun 27, 2009 at 6:07 PM, Edward Ing edward@gmail.com wrote: saveFile n = do cont - (liftM fromJust) $ getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f cont return $ paragraph (Saved as +++ anchor ! [href f] f +++ .) Vs. saveFile n = do cont - getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f (fromJust cont) return $ paragraph (Saved as +++ anchor ! [href f] f +++ .) Consider the line x - y in a do expression. If y has type M a for some monad M, then x has type a. So, let's say you have a value f :: Maybe Int, and you want to return the Int's stringification if it exists. We can write this in these two ways: do x - f return (show x) do x - liftM show f return x liftM :: (a - b) - (M a - M b) for any monad M. That means if you want to apply a function to a value which is currently wrapped in a monad constructor, you need to lift it in. liftM takes a function on ordinary values to a function on wrapped values. But *after* you bind, you don't need to lift anymore. Which of the two above styles to choose is a matter of style, and, in my code at least, varies from situation to situation. That said, you can write both of these snippets as fmap show f or show $ f (where ($) is from Control.Applicative), which is how it would be done in practice. Does that make sense? Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Network.CGI -- practical web programming example.
On Jun 27, 2009, at 20:07 , Edward Ing wrote: saveFile n = do cont - (liftM fromJust) $ getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f cont return $ paragraph (Saved as +++ anchor ! [href f] f +++ .) saveFile n = do cont - getInputFPS file let f = uploadDir ++ / ++ basename n liftIO $ BS.writeFile f (fromJust cont) return $ paragraph (Saved as +++ anchor ! [href f] f + ++ .) 1) Why did the author choose to insert liftM in function saveFile? It's because of where fromJust is being called. In yours, it's being used at a place that expects a normal value, so you can just go ahead and use it. The original is applying the fromJust inside of a monadic computation, as indicated by the (-), so it needs to be lifted. Some Haskell programmers use fmap (because most Monads are also Functors), others use liftM. Both have the same effect: given a monadic computation m a, liftM f turns f into a function that operates on the enclosed a instead of the entire m a. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] bizarre syntax error
I ran into a unfortunate syntax error just now. I figured I'd share it because it's the weirdest message I've ever gotten out of ghc. The broken code is case t of TyApply tv types - do (tvl, cases) - lookupDatatype prog tv let tenv = Map.fromList (zip tvl types) -- GHC POINTS HERE caseType (c,vl,e') | Just tl - List.lookup c cases = if length vl == a then expr prog global (foldl (\e (v,t) - Map.insert v t e) env (zip vl (map (subst tenv) tl))) e' else typeError (arity mismatch in pattern: ++show (pretty c)++ expected ++show a++ argument++(if a == 1 then else s) ++ but got [++concat (intersperse , (map (show . pretty) vl))++]) where a = length tl -- THIS IS THE PROBLEM | otherwise = typeError (datatype ++show (pretty tv)++ has no constructor ++show (pretty c)) defaultType Nothing = return [] defaultType (Just (v,e')) = expr prog global (Map.insert v t env) e' =. \t - [t] join t1 t2 | Just t - unifyS t1 t2 = return t | otherwise = typeError (failed to unify types ++show (pretty t1)++ and ++show (pretty t2)++ from different case branches) caseResults - mapM caseType pl defaultResults - defaultType def foldM1 join (caseResults ++ defaultResults) _ - typeError (expected datatype, got ++show (pretty t)) ghc complains that The last statement in a 'do' construct must be an expression and points to the line declaring tenv. The actual problem is that you can't put a where block in between two pattern guards. :) Geoffrey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] bizarre syntax error
On Jun 27, 2009, at 20:37 , Geoffrey Irving wrote: The last statement in a 'do' construct must be an expression Right, the where terminates the entire expression, you can't use it in the middle like that. I'm pretty sure that's mandatory per the standard, and nobody really wants to open the can of worms involved with allowing its nested use as an extension. (Layout is hard enough to parse already; IIRC, strictly speaking, the behavior specified in the standard is impossible to implement, and even almost right is extremely difficult.) Use a subsidiary let instead: caseType (c,vl,e') | Just tl - let a = length tl in List.lookup c cases = if length vl == a then expr prog global (foldl (\e (v,t) - Map.insert v t e) env (zip vl (map (subst tenv) tl))) e' else typeError (arity mismatch in pattern: ++show (pretty c)++expected++show a++ argument++(if a == 1 then else s) ++ but got [++concat (intersperse , (map (show . pretty) vl))++]) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] test-framework success
Max - I was thinking about how to drive my new shell tests with your
framework. I have:
data ShellTest = ShellTest {
filename ∷ String
,command ∷ String
,stdin∷ Maybe String
,stdoutExpected ∷ Maybe String
,stderrExpected ∷ Maybe String
,exitCodeExpected ∷ Maybe ExitCode
} deriving (Show)
parseShellTest ∷ FilePath → IO ShellTest
runShellTest ∷ ShellTest → IO Bool
I converted to hunit tests with
shellTestToHUnitTest t = filename t ~: do {r ← runShellTest t;
assertBool r}
and tied test-framework into my test runner with
main = do
args ← getArgs
let (opts,files) = partition ((==-).take 1) args
hunittests ← mapM (λf → parseShellTest f = (return ∘
shellTestToHUnitTest)) files
withArgs opts $ defaultMain $ concatMap hUnitTestToTests hunittests
With surprisingly little code and pain, the new runner runs my 14
shell tests, with colored output, in the same time as the old one -
3.5s on this macbook.
But, I can now add -j8 and get the same results output in.. 0.13s.
This quite surprised me, and now I want to say: thank you very much! :)
Best,
-Simon
$ time tools/shelltest2 tests/*.test -j8
:tests/eliding-balance.test: [OK]
:tests/missing-real-and-virtual-amt.test: [OK]
:tests/null-accountname-component.test: [OK]
:tests/parens-in-account-name.test: [OK]
:tests/sample-balance-depth.test: [OK]
:tests/sample-balance-o.test: [OK]
:tests/sample-balance.test: [OK]
:tests/unbalanced.test: [OK]
:tests/unicode-account-matching.test: [OK]
:tests/unicode-balance.test: [OK]
:tests/unicode-description-matching.test: [OK]
:tests/unicode-print.test: [OK]
:tests/unicode-register.test: [OK]
:tests/virtual.test: [OK]
Test Cases Total
Passed 14 14
Failed 00
Total 14 14
real0m0.132s
user0m0.104s
sys 0m0.104s
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[Haskell-cafe] Re: test-framework success
PS - not cabalised, not even committed, but here's my shell test runner for folks to play with: http://joyful.com/repos/hledger/tools/shelltest2.hs http://joyful.com/repos/hledger/tests/ - test examples ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Rigid type variable error
Thanks for the help previously received, but I still cannot seem to get
on top of this. The types for the constructor K will not resolve and
I'm at a loss to work out what to do with it. If anyone can offer
a further explanation and help I would be very grateful.
My code (File Test5.hs):
{-# LANGUAGE ExistentialQuantification #-}
class A a where
a1 :: a
a2 :: a - a
a3 :: (B b) = b - a
class B b where
b1 :: Int - b
--data Ainst b = I | J (Ainst b) | K b
-- a3 :: (B b, A a) = b - a
-- yet without the constraint on K, K :: b - Ainst b
-- so the above data definition fails. Trying to
-- existentially quantify K below seems to make
-- sense, but also fails ...
data Ainst b = I | J (Ainst b) | (B b) = K b
instance (B b) = A (Ainst b) where
a1 = I
a2 = J
a3 = K -- Reported line of the error
data Binst = Val Int
instance B Binst where
b1 = Val
---
The error from ghci is as follows:
Test5.hs:25:9:
Couldn't match expected type `b' against inferred type `b1'
`b' is a rigid type variable bound by
the type signature for `a3' at Test5.hs:7:13
`b1' is a rigid type variable bound by
the instance declaration at Test5.hs:16:12
Expected type: b - Ainst b1
Inferred type: b1 - Ainst b1
In the expression: K
In the definition of `a3': a3 = K
Failed, modules loaded: none.
Thanks in advance for any help. Apologies if what I am doing is odd or
the answer is obvious, I'm still very new to Haskell.
Darryn.
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