- Glasgow-haskell-users

Data.Typeable and default instances
by Jim Apple 04 Dec '05

04 Dec '05

> {-# OPTIONS -fglasgow-exts #-} > > import Maybe > import Data.Typeable > > data Nil = Nil deriving (Eq,Typeable,Show) > > class (Typeable t) => List a t where > init :: (t -> b) -> (forall y . (List a y) => y -> b) > init f z = fromJust $ do x <- cast z > return $ f x > > instance List a Nil where Could not deduce (List a1 y) from the context (List a Nil, Typeable Nil, List a y) arising from use of `Main.$dminit' at Main.hs:21:0 Probable fix: add (List a1 y) to the class or instance method `Main.init' In the definition of `init': init = Main.$dminit In the definition for method `Main.init' In the instance declaration for `List a Nil' but copying and pasting the code from init to the instance declaration works fine. Jim

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GHCI and archive libraries.
by Keean Schupke 04 Dec '05

04 Dec '05

GHCI does not load archive libraries. Is it possible (easy?) to get it to load (.a) archive libraries as well as .o and .so files? The problem is some optimized "cblas" libraries are not available as shared libraries due to the performace loss. Regards, Keean.

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exhaustive pattern warning bug with irrefutable patterns
by John Meacham 04 Dec '05

04 Dec '05

the following gives a 'non-exhaustive pattern matching' error when it shouldn't. the ~-pattern always matches. It is not the function that is non-exhausitive, but the irrefutable binding, which are necessarily always non-exhausive so warning about it is the wrong thing to do. f :: [a] -> a f [x] = x f ~(_:xs) = f xs John -- John Meacham - ⑆repetae.net⑆john⑈

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ghc --make and make
by John Meacham 04 Dec '05

04 Dec '05

I was thinking it would be nice if ghc --make could touch the output file with the timestamp of the most recently modified source file. As it is, if you edit a file in between when ghc --make starts and it finishes (a substantial amount of time in some cases) then 'make' will not realize the dependencies have changed. John -- John Meacham - ⑆repetae.net⑆john⑈

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RE: -fwarn-incomplete-patterns, not always warns
by Simon Peyton-Jones 02 Dec '05

02 Dec '05

It's a GHC weakness. I've added your example to the currently-open bug for it: http://sourceforge.net/tracker/index.php?func=detail&aid=1075259&group_i d=8032&atid=108032 Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Neil Mitchell | Sent: 01 December 2005 20:56 | To: glasgow-haskell-users(a)haskell.org | Subject: -fwarn-incomplete-patterns, not always warns | | Hi, | | I have been playing around with -fwarn-incomplete-patterns under GHC | 6.4.1 on Windows. | | -- no warning | ex1 x = ss | where (s:ss) = x | | -- no warning | ex2 x = let (s:ss) = x in ss | | -- Warning: Pattern match(es) are non-exhaustive | -- In a case alternative: Patterns not matched: [] | ex3 x = case x of ~(s:ss) -> ss | | I have translated all 3 functions using the rules supplied in the | Haskell 98 report, so they all have the same meaning, but only one | gives an error. Is it intentional to ignore where/let pattern matches? | | Thanks | | Neil | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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-fwarn-incomplete-patterns, not always warns
by Neil Mitchell 01 Dec '05

01 Dec '05

Hi, I have been playing around with -fwarn-incomplete-patterns under GHC 6.4.1 on Windows. -- no warning ex1 x = ss where (s:ss) = x -- no warning ex2 x = let (s:ss) = x in ss -- Warning: Pattern match(es) are non-exhaustive -- In a case alternative: Patterns not matched: [] ex3 x = case x of ~(s:ss) -> ss I have translated all 3 functions using the rules supplied in the Haskell 98 report, so they all have the same meaning, but only one gives an error. Is it intentional to ignore where/let pattern matches? Thanks Neil

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RE: Associated types in 6.6?
by Simon Peyton-Jones 29 Nov '05

29 Nov '05

| Is that because GHC's TIL is not exactly System F? It's an extension of System F (e.g. with data types, existentials, GADTs). But we're not sure it's the right extension yet. Stay tuned. | > As ever, we tend to work harder on things that folk appear to want; | | Unrelated question: will "boxy" types allow forall-quantified types in | instance declarations? Have not thought about that; do offer examples of how useful it'd be. Simon

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RE: Associated types in 6.6?
by Simon Peyton-Jones 29 Nov '05

29 Nov '05

| I see that associated types is already in CVS: | | http://article.gmane.org/gmane.comp.lang.haskell.cvs.all/19423/match=ass ociated | | Will it be in 6.6? I hope so but I am not sure. The trick lies in coming up with a suitable typed intermediate representation for the program -- System F isn't enough. Manuel Chakravarty and Martin Sulzmann and I have been working on an idea, but it's not yet clear whether we'll have it worked out and implemented at the same time as 6.6. As ever, we tend to work harder on things that folk appear to want; so anyone who is keen on associated types, do sing out and describe your application a bit. Simon

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calling system gives exception, waitForProcess
by Neil Mitchell 28 Nov '05

28 Nov '05

Hi, On Windows XP, with GHC 6.4.1, I get the following error consistently when calling "system", from the module System. I get this error on two entirely different computers, one slow (750Mhz) system, and one fast (3Ghz) system. Its easiest to demonstrate when using GHCi, but the problem also occurs when running Haskell programs compiled with GHC. Prelude System> system "yhi Main" PreludeList.!!: on empty list *** Exception: waitForProcess: does not exist (No such file or directory) >From a windows standard console, in the same current directory, I get: Windows> yhi Main PreludeList.!!: on empty list When calling yhi using system in other circumstances (on other programs), it works fine. The "PreludeList.!! ..." text is being generated by the yhi program. The yhi program is written in C, and is available from http://www.cs.york.ac.uk/~ndm/yhc/. Unfortunately this is probably a bit too large to use as a minimal test case, and takes quite a while to install/configure at the moment. Any idea why the waitForProcess error is being raised? Thanks Neil

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RE: Rebindable monads?
by Simon Peyton-Jones 28 Nov '05

28 Nov '05

| Where is there documentation for rebindable syntax for monads with class | constraints: | | (>>=) :: (Foo m, Baz a) => m a -> (a -> m b) -> m b | | http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/9447/match=synt ax | | The users guide seems to disallow such type signatures: | | http://haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#re bindable-syntax I don't think it's in 6.4. Here's the documentation from the HEAD. Simon 7.3.5. Rebindable syntax GHC allows most kinds of built-in syntax to be rebound by the user, to facilitate replacing the Prelude with a home-grown version, for example. You may want to define your own numeric class hierarchy. It completely defeats that purpose if the literal "1" means "Prelude.fromInteger 1", which is what the Haskell Report specifies. So the -fno-implicit-prelude flag causes the following pieces of built-in syntax to refer to whatever is in scope, not the Prelude versions: * An integer literal 368 means "fromInteger (368::Integer)", rather than "Prelude.fromInteger (368::Integer)". * Fractional literals are handed in just the same way, except that the translation is fromRational (3.68::Rational). * The equality test in an overloaded numeric pattern uses whatever (==) is in scope. * The subtraction operation, and the greater-than-or-equal test, in n+k patterns use whatever (-) and (>=) are in scope. * Negation (e.g. "- (f x)") means "negate (f x)", both in numeric patterns, and expressions. * "Do" notation is translated using whatever functions (>>=), (>>), and fail, are in scope (not the Prelude versions). List comprehensions, mdo (Section 7.3.3, "The recursive do-notation " <file:///\\Cam-01-unx\simonpj\builds\HEAD-1\ghc\docs\users_guide\users_g uide\syntax-extns.html#mdo-notation> ), and parallel array comprehensions, are unaffected. * Arrow notation (see Section 7.7, "Arrow notation " <file:///\\Cam-01-unx\simonpj\builds\HEAD-1\ghc\docs\users_guide\users_g uide\arrow-notation.html> ) uses whatever arr, (>>>), first, app, (|||) and loop functions are in scope. But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask! In all cases (apart from arrow notation), the static semantics should be that of the desugared form, even if that is a little unexpected. For emample, the static semantics of the literal 368 is exactly that of fromInteger (368::Integer); it's fine for fromInteger to have any of the types: fromInteger :: Integer -> Integer fromInteger :: forall a. Foo a => Integer -> a fromInteger :: Num a => a -> Integer fromInteger :: Integer -> Bool -> Bool Be warned: this is an experimental facility, with fewer checks than usual. Use -dcore-lint to typecheck the desugared program. If Core Lint is happy you should be all right.

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