- Glasgow-haskell-users

RE: inscrutable warning
by Simon Peyton-Jones 08 Feb '02

08 Feb '02

It's because there's an application of a strict function, the dictionary selector 'a'. In general, if f is strict then f (g x) is turned into case g x of y { DEFAULT -> f y } But it is rather inscrutable. It didn't arise from a strictness annotation put in by the user. The right thing to do is to fix the bytecode generator. Simon | -----Original Message----- | From: Julian Seward (Intl Vendor) | Sent: 08 February 2002 13:55 | To: 'Dean Herington'; glasgow-haskell-users(a)haskell.org | Cc: Simon Peyton-Jones | Subject: RE: inscrutable warning | | | | The warning says that the interpreter has ignored a | polymorphic case, since it's hard to implement. Your program | should still behave the same, although possibly less strictly | than you intended. | | What really mystifies us is why there is such a thing in the | program in the first place. Using -ddump-sat we can see that | there is indeed a | case ... of { DEFAULT -> ... } | but how it came to be, I do not know. | | We may investigate further. | | J | | | Can someone explain the following warning? | | | | ------------------------------- | | module Warning where | | | | type A = IO () | | | | class CA t | | where a :: t -> A | | instance CA () | | where a = return | | instance (CA t) => CA (IO t) | | where a = (>>= a) | | | | data B = B A | | | | class CB t | | where b :: t -> B | | instance (CA t) => CB (IO t) | | where b = B . a | | ------------------------------- | | | | % ghci Warning.hs | | ___ ___ _ | | / _ \ /\ /\/ __(_) | | / /_\// /_/ / / | | GHC Interactive, version 5.02.2, | | for Haskell | | 98. | | / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | | \____/\/ /_/\____/|_| Type :? for help. | | | | Loading package std ... linking ... done. | | Compiling Warning ( Warning.hs, interpreted ) | | WARNING: ignoring polymorphic case in interpreted mode. | | Possibly due to strict polymorphic/functional constructor args. | | Your program may leak space unexpectedly. | | | | Ok, modules loaded: Warning. | | Warning> | | | | _______________________________________________ | | Glasgow-haskell-users mailing list | | Glasgow-haskell-users(a)haskell.org | | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users | | |

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RE: inscrutable warning
by Julian Seward (Intl Vendor) 08 Feb '02

08 Feb '02

The warning says that the interpreter has ignored a polymorphic case, since it's hard to implement. Your program should still behave the same, although possibly less strictly than you intended. What really mystifies us is why there is such a thing in the program in the first place. Using -ddump-sat we can see that there is indeed a case ... of { DEFAULT -> ... } but how it came to be, I do not know. We may investigate further. J | Can someone explain the following warning? | | ------------------------------- | module Warning where | | type A = IO () | | class CA t | where a :: t -> A | instance CA () | where a = return | instance (CA t) => CA (IO t) | where a = (>>= a) | | data B = B A | | class CB t | where b :: t -> B | instance (CA t) => CB (IO t) | where b = B . a | ------------------------------- | | % ghci Warning.hs | ___ ___ _ | / _ \ /\ /\/ __(_) | / /_\// /_/ / / | | GHC Interactive, version 5.02.2, | for Haskell | 98. | / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | \____/\/ /_/\____/|_| Type :? for help. | | Loading package std ... linking ... done. | Compiling Warning ( Warning.hs, interpreted ) | WARNING: ignoring polymorphic case in interpreted mode. | Possibly due to strict polymorphic/functional constructor args. | Your program may leak space unexpectedly. | | Ok, modules loaded: Warning. | Warning> | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users |

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Unsure about 'IO'
by Alex van Ballegooij 06 Feb '02

06 Feb '02

Dear all, We've been working on bulding an interpreter in haskell and we seem to have encountered a problem. At the end of the email I've pasted a minimal version of the program, that shows the error. The problem is that we 'putStr' a prompt onto (unbuffered) stdout and then read a line, process it and 'print' output, all very basic. This is done in the function 'processline' and works fine is this function is used directly from 'main', however, if used from within the 'while' construct it fails. In the 'while' context it appears that the 'getLine' is executed before the 'putStr' is (or perhaps stdout is not as 'unbuffered' as we expect ?). As all the functions in this example are sequenced in the context of the IO monad, I guess that order of execution should be predefined, right ? Any help would be greatly appreciated, just for information: we tested with GHC 5.02.1 and 5.02.2. Kind regards, Alex... ---- import IO import Monad notM :: Monad m => m Bool -> m Bool notM = liftM not processline :: IO () processline = do putStr "prompt: " line <- getLine putStrLn ("Read: " ++ line) while :: Monad m => m Bool -> m a -> m () while t s = do b <- t if(b) then do { s; while t s; return () } else do { return () } main :: IO () main = do hSetBuffering stdout NoBuffering putStrLn "Single call:" processline putStrLn "Call from 'while' construct:" while (notM isEOF) (processline)

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RE: Unsure about 'IO'
by Simon Marlow 06 Feb '02

06 Feb '02

> We've been working on bulding an interpreter in haskell > and we seem to have encountered a problem. > At the end of the email I've pasted a minimal version of the program, > that shows the error. > > The problem is that we 'putStr' a prompt onto (unbuffered) stdout > and then read a line, process it and 'print' output, all very basic. > > This is done in the function 'processline' and works fine is this > function is used directly from 'main', however, if used from within > the 'while' construct it fails. > In the 'while' context it appears that the 'getLine' is executed > before the 'putStr' is (or perhaps stdout is not as 'unbuffered' > as we expect ?). In your example it is the 'isEOF' that is waiting for input. The I/O system can't determine whether the end of the input has been reached yet without waiting for the user to type something: the user might type ^D which would indicate EOF, or they might type some characters. This is arguably wrong, although it does seem to follow the Haskell Library Report (section 11.4.1). Strictly speaking, it would be wrong not to block in isEOF because we might be at the end of file and not know it yet. You can use hReady to determine whether isEOF will block (although interestingly the report says that hReady can fail with an EOF error; GHC won't do this in practice). Cheers, Simon

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inscrutable warning
by Dean Herington 06 Feb '02

06 Feb '02

Can someone explain the following warning? ------------------------------- module Warning where type A = IO () class CA t where a :: t -> A instance CA () where a = return instance (CA t) => CA (IO t) where a = (>>= a) data B = B A class CB t where b :: t -> B instance (CA t) => CB (IO t) where b = B . a ------------------------------- % ghci Warning.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.02.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package std ... linking ... done. Compiling Warning ( Warning.hs, interpreted ) WARNING: ignoring polymorphic case in interpreted mode. Possibly due to strict polymorphic/functional constructor args. Your program may leak space unexpectedly. Ok, modules loaded: Warning. Warning>

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RE: ANNOUNCE: GHC 5.03.20020204 snapshot release
by Simon Peyton-Jones 06 Feb '02

06 Feb '02

| > - newtypes support deriving *any* class for which the | > underlying type is also an instance. | | How about multi-parameter classes where there are instances for the | underlying type on more than one parameter: is it possible to specify | which parameter the newtype is being derived on? There's a fairly full description in the manual. | > - Linear implicit parameters: a highly experimental feature. | | What are they? Aha. That would be telling. The syntax is %x instead of ?x. Simon

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compile-time evaluation
by David Feuer 06 Feb '02

06 Feb '02

>Just saying >'evaluate this CAF to WHNF' might only inline the outer combinator. I'm not sure quite what you mean. However, it seems to me that this sort of thing would match the way other things happen in Haskell fairly well (i.e., default lazy, strictify as necessary). If you want something to be evaluated more deeply, make a CAF to do so, and then tell the compiler to evaluate that... I suspect even this very limited control would be useful for some applications that would otherwise have long and useless start-up times. On a more adventurous note, I think it would be very interesting to be able to ask the compiler to make certain values persistent across program executions. This could solve the same problem, but may also help deal with long computations that may be interrupted, and should be re-started at the same point. (actually, I think this would not be all that great... however, if non-trivial memo tables ever get added to GHC, it could be _very_ useful to store those across program runs for simulations... that way, if the program is given input 12, and told to calculate something with it and print it out, if the program is interrupted, and re-started with the same input, calculation could presumably start at the point it left off...). Of course, I have no clue how realistic any of this is. David Feuer This message has been brought to you by the letter alpha and the number pi.

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Boxing parameterized datatypes?
by Scott Williams 06 Feb '02

06 Feb '02

This one may be fairly basic, but I was wondering if there's a common idiom for boxing a parameterized datatype into a non-parameterized one, and accessing the components in a unified manner. (hope I said that right, pretend I mentioned polymorphism somewhere) > module VarExample where For example if I create a datatype to hold everything about a variable > data Base t = > Base { value :: t > , name :: String > -- , ... more components > } that I want to use as part of a larger data structure, say a heterogeneous list or an AST, I think I have to collect all of the variations under a parameterless datatype declaration. Is this true? > data Var = > IntegerT (Base Integer) > | DoubleT (Base Double) > -- | ... more constructors > type HeterogeneousList = [Var] If so, it looks like I then need to write something like the following in order to access the name of a given Var, and likewise for other components. > getName :: Var -> String > getName (IntegerT b) = name b > getName ( DoubleT b) = name b Doesn't seem bad, but if I have more components and type constructors it can become unwieldy. The following give compile-time errors, but represent what I've tried so far to simplify things. > getBase :: Var -> Base t > getBase (IntegerT b) = b > getBase ( DoubleT b) = b > getName2 v = name (getBase v) > callBaseFn :: Var -> (Base t -> a) -> a > callBaseFn (IntegerT b) fn = fn b > callBaseFn ( DoubleT b) fn = fn b > getName3 v = callBaseFn v name The other option I've thought about is to use the GHC Dynamic module, but it doesn't seem very standard for what there is likely a common idiom. As you can see I'm still somewhat new at this, but would appreciate any help. - Thanks, Scott Williams

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RE: Binary Compatibility
by Julian Seward (Intl Vendor) 06 Feb '02

06 Feb '02

| -----Original Message----- | From: Sigbjorn Finne [mailto:sof@galois.com] | Sent: Wednesday, February 06, 2002 1:27 AM | To: Ashley Yakeley | Cc: GHC List | Subject: Re: Binary Compatibility | | | > Apologies if this has been covered before. What | compatibility is there | > between code compiled in different versions of GHC? | > | | As a rule, none. Always recompile your code for every new | release. Yes, even patchlevel ones. For your specific case we know for sure that the code produced by 5.02 is incompatible with both 5.02.2 and 5.03. J

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Binary Compatibility
by Ashley Yakeley 06 Feb '02

06 Feb '02

Apologies if this has been covered before. What compatibility is there between code compiled in different versions of GHC? My JVM-Bridge was compiled under 5.02 and assembled as package 'javavm'. Will that package work if added to GHC 5.02.2? What about 5.03? -- Ashley Yakeley, Seattle WA

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