October 2007 - Haskell-Cafe

EnableGUI hack changes working directory
by apfelmus 09 Oct '07

09 Oct '07

Hello, the EnableGUI hack for getting a window in GHCi on Mac OS X (10.3.9 for me) unexpectedly changes the working directory for the running ghci [...] apfelmus$ ghci -i../../Hackage/SOE/src [...] GHC Interactive, version 6.6.1, for Haskell 98. *Main> :! pwd /Users/apfelmus/Documents/Programmierung/Haskell/Graphics/Kunst *Main> enableGUI >> run lambda Loading package OpenGL-2.2.1 ... linking ... done. Loading package GLFW-0.1 ... linking ... done. *Main> :! pwd /opt/local/lib/ghc-6.6.1 The effect is that reloading the Main module now fails *Main> :r <no location info>: can't find file: Main.hs Any solution? Btw, using enableGUI exactly once at the beginning of the ghci session doesn't work *Main> enableGUI <interactive>: ../../Hackage/SOE/src/EnableGUI.o: unknown symbol `_CPSEnableForegroundOperation' unless ghci is started with the -framework Carbon option. But then, the working directory remains fine, so I guess the change of working directory bug happens while dynamically loading frameworks/shared libraries. Is there a way to automate ghci ... EnableGUI -framework Carbon and typing enableGUI at the first prompt? Something like a custom shell script ghci+gui or even ghci --with-gui ? Regards, apfelmus

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unsafePerformIO: are we safe?
by Jorge Marques Pelizzoni 09 Oct '07

09 Oct '07

Hi, all! This is a newbie question: I sort of understand what unsafePerformIO does but I don't quite get its consequences. In short: how safe can one be in face of it? I mean, conceptually, it allows any Haskell function to have side effects just as in any imperative language, doesn't it? Doesn't it blow up referential transparency for good? Is there anything intrinsic to it that still keeps Haskell "sound" no matter what unsafePerformIO users do (unlikely) or else what are the guidelines we should follow when using it? Thanks in advance. Cheers, Jorge.

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Type-level arithmetic
by Andrew Coppin 09 Oct '07

09 Oct '07

I've seen quite a few people do crazy things to abuse the Haskell type system in order to perform arithmetic in types. Stuff the type system was never ever intended to do. Well I was just wondering... did anybody ever sit down and come up with a type system that *is* designed for this kind of thing? What would that look like? (I'm guessing rather complex!)

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ANNOUNCE: binary 0.4: high performance, pure binary parsing and serialisation
by Don Stewart 08 Oct '07

08 Oct '07

Binary: high performance, pure binary encoding, decoding and serialisation for Haskell ---------------------------------------------------------------------- The Binary Strike Team is pleased to announce release 0.4 of Data.Binary, the pure, efficient binary serialisation library for Haskell, now available from Hackage: tarball: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary/0.4 darcs: darcs get http://darcs.haskell.org/binary The 'binary' package provides efficient serialisation of Haskell values to and from lazy ByteStrings, and a low level layer for high performance decoding and decoding binary data. ByteStrings constructed this way may then be written to disk, written to the network, or further processed (e.g. stored in memory directly, or compressed in memory with zlib or bzlib). *Very* high performance can be expected, with throughput over 1G/sec observed in practice (good enough for most networking scenarios, we suspect). Encoding and decoding are achieved by the functions: encode :: Binary a => a -> ByteString decode :: Binary a => ByteString -> a which mirror the read/show functions. Convenience functions for serialising to disk are also provided: encodeFile :: Binary a => FilePath -> a -> IO () decodeFile :: Binary a => FilePath -> IO a To serialise your Haskell data, all you need do is write an instance of Binary for your type. For example, suppose in an interpreter we had the data type: import Data.Binary import Control.Monad data Exp = IntE Int | OpE String Exp Exp We can serialise this to bytestring form with the following instance: instance Binary Exp where put (IntE i) = putWord8 0 >> put i put (OpE s e1 e2) = putWord8 1 >> put s >> put e1 >> put e2 get = do tag <- getWord8 case tag of 0 -> liftM IntE get 1 -> liftM3 OpE get get get The binary library has been heavily tuned for performance, particularly for writing speed. On average, Data.Binary is 10x faster than NewBinary, and has the advantage of a pure interface, and bytestring return values. Binary was developed by a team of 8 during the Haskell Hackathon, Hac 07, in January 2007, and this maintainence release has taken place during the second hackathon. Binary is portable, using the foreign function interface and cpp, and has been tested with Hugs and GHC. Happy hacking! -- Don

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Confusing type specialisation in ghci
by David Carter 08 Oct '07

08 Oct '07

Hello, Can someone elucidate to me this behaviour from ghci 6.6.1? Why is the type of sqlist specialised to Integer? Prelude> let sq x = x * x Prelude> :t sq sq :: (Num a) => a -> a Prelude> sq 2.5 6.25 Prelude> :t map sq map sq :: (Num a) => [a] -> [a] Prelude> map sq [2.5] [6.25] This is all as I would expect so far, but: Prelude> let sqlist = map sq Prelude> :t sqlist sqlist :: [Integer] -> [Integer] And indeed, I get Prelude> sqlist [2.5] <interactive>:1:8: No instance for (Fractional Integer) ... etc Thanks! David -- The Wellcome Trust Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.

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Re: [Haskell] Converting Double from String
by Brent Yorgey 08 Oct '07

08 Oct '07

(moving to haskell-cafe...) On 10/8/07, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > On 10/8/07, bjornie <bjorn.wikstrom(a)gmail.com> wrote: > > > > Hi! I'm kind of new to functional programming with Haskell, I'm reading > a > > university course you see. Now I'm having some problems with one of our > lab > > assignments, we're supposed to write a program handling numeric > operations > > (addition, multiplication, and some functionality like sin and cos). > > > > I've made a data type; > > data Expr = Num Double | Add Expr Expr | Mul Expr Expr... > > > > I.e for representing the expression: > > 2.4 * (4.0 + 1.5) > > The Haskell representation is: > > Mul (Num 2.4) (Add (Num 4.0) (Num 1.5)) > > > > My problem is to convert a String to an expression like the one above. > > I.e the String: "2.4*(4.0+1.5)" > > > > If we would use plain Integers there would not be a problem, since the > > Prelude function 'digitToInt' :: Char -> Int does nearly what I would > like > > to do. But I'm stuck on the Doubles! How can I solve this in a smooth > way? > > You can use read. Here's a session from GHC's interpreter: > > $ ghci > Prelude> read "1.0" :: Double > 1.0 > > Since read is polymorphic (i.e. works for different types) you need to > specify the type of the result you want using ":: Double". ...although if you have something like Num (read x), you don't need the :: Double since the typechecker can figure it out (the Num constructor has to take a Double). Note that you would use the 'read' function to read integer values, too; digitToInt is just for dealing with single Chars, not entire integral values. The read function takes a String representation of anything that is "readable" (any type that is an instance of the Read type class) and converts it into a value of the requested type, so it works for Doubles, Ints, Integers, lists, and so on... Also, if I were you I'd use something other than "Num" as the name of the data constructor for numeric values in your Expr type. Num is already used for something else (it's the name of the general numeric type class), and although it's legal to have the same name for both, it might lead to confusion, especially when dealing with error messages. Good luck with learning Haskell and with your course! If you need more help there's also the #haskell IRC channel on freenode.net<http://haskell.org/haskellwiki/IRC_channel>, where you'll find lots of friendly people who would be glad to answer your questions. -Brent

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Data types, opengl display loop and readIORef/writeIORef
by bbrown 08 Oct '07

08 Oct '07

This is more an aesthetics question, I have a simple opengl application that has a singleton like object I need to pass around to the display loop and possibly to the keyboard action handler. (pseudo code) data SimpleMech = SimpleMech { mechPos :: !MVector, mechRot :: !MVector } deriving (Read) mainMech = initMech (0, 0, 0) (0, 0, 0) mainMechRef <- newIORef(mainMech) displayCallback $= displayGameLoop mainMechRef rotateMech :: (Double, SimpleMech) -> SimpleMech rotateMech (angle, mech) = -- Calculate a new rotation let (xrot,yrot,zrot) = (mechRot mech) newyrot = newyrot + angle in SimpleMech { mechRot = (xrot, newyrot, zrot), mechPos = (mechPos mech) } displayGameLoop mechRef = do mech <- get mechRef mechRef $= (rotateMech (0.1, mech)) ... ... For some reason in that code above, the "mechRef" doesnt update the value I want to change. It is like I am getting an instance of the initial valyue back. Also, when I use writeIORef, eg writeIORef mechRef (rotateMech (0.1, mech) I get stack overflow exceptions. The displayGameLoop will get called an infinite amount of times, so I think I have to be careful there. But I wonder why the reference isn't getting updated? Is this an approach you would take, eg passing "mechRef" around, a singleton game object. -- Berlin Brown [berlin dot brown at gmail dot com]

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cabal library dependencies
by Nehir Sonmez 08 Oct '07

08 Oct '07

Hi All, To test some programs written in CABAL, I need to use a server (where I do not have root access) with GHC 6.6.1 (in-place) installed. However, many programs need more libraries that have to be installed, (such as binary, or derive) for which, even when I use ./Setup configure --prefix=something, 1) After the ./Setup build, it can not do ./Setup install correctly because I am not root, ie. Installing: /usr/local/lib/derive-0.1/ghc-6.6.1 & /usr/local/bin derive-0.1... Setup: /usr/local/lib/derive-0.1: createDirectory: permission denied (Permission denied) 2) But the .o and the .a files are created. However possibly due to (1), I can not make the program see the library, so in the end when I try configuring the program, I get (for example): Setup: cannot satisfy dependency derive-any possible solutions I could think of were: 1) LD_LIBRARY_PATH 2) use configure with a --with flag 3) copying the library files to the prefix directory of the program I ultimately want to run 4) The -L flag and still no luck, so here I am asking for your help... Thanks a lot in advance, Sincerely, -Nehir Sonmez

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Re: [Haskell] reading from stdin
by Reinier Lamers 08 Oct '07

08 Oct '07

Axel Simon wrote: >a) is the behaviour I want, but unfortunately for platform d) >b) must be due to ghci and Hugs having different opinions on whether >stdin should be line buffered or unbuffered >c) this is weird >d) this is broken > You can probably unify the behaviors of platforms a, b and c by calling hSetBuffering on stdin to turn off buffering. Reinier

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Typechecker to GADT: the full implementation of a typed DSL
by oleg＠pobox.com 08 Oct '07

08 Oct '07

Pasqualino 'Titto' Assini wrote: > I am trying to write an interpreter for a little functional language but I am > finding very problematic to dynamically create a typed representations of the > language terms. > > > The problem is to write a function that converts between Exp and Term > > t as in: > > > > > test :: Term Double > > > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) The specification leaves out a few important details. The typechecker can't be a total function: what happens when typechecking this code? EApp (EDouble 10.0) (EPrim "inc") The error has to be reported somehow, and it is important to know how. There are basically two choices. One is to keep the above interface > test :: Term Double > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) which means using Template Haskell and means letting TH reporting type errors (with no hope of catching them). The second choice is `typing dynamic typing'. Which means we can't write a typechecker with the signature "Exp -> Term t". Rather, we have to write typecheck :: Exp -> (forall t. Typ t -> Term t -> w) -> Maybe w or, equivalently, using an existential data MostlyStatic = forall t. MostlyStatic (Typ t, Term t) typecheck :: Exp -> Maybe MostlyStatic Although both MostlyStatic and Exp are `untyped', the latter is `deeply' untyped, whereas the former is only untyped at the surface. In the case of MostlyStatic, the term is built out of typed components, and the type is erased only at the end. These choices aren't contradictory: using TH, we can convert MostlyStatic to the proper Haskell term. Because the term has already been typechecked, we are guaranteed the absence of errors during such a conversion. For today, perhaps the implementation of the second choice will be sufficient. This is the complete representation of a typed DSL given in the untyped AST form. We typecheck a term. We either report a type error, or evaluate the typed term and then report result, if can be shown. We also show the inferred type of the result. Examples. Let's assume the following terms (not all of them well-typed) te1 = EApp (EPrim "inc") (EDouble 10.0) te2 = EApp (EDouble 10.0) (EPrim "inc") te3 = EApp (EApp (EPrim "+") (EApp (EPrim "inc") (EDouble 10.0))) (EApp (EPrim "inc") (EDouble 20.0)) te4 = EApp (EPrim "rev") te3 te5 = EApp (EPrim "rev") (EApp (EPrim "show") te3) *Eval> teval te1 "type Double, value 11.0" *Eval> teval te2 "Type error: Trying to apply not-a-function: Double" *Eval> teval te3 "type Double, value 32.0" *Eval> teval te4 "Type error: incompatible type of the application: (String->String) and Double" *Eval> teval te5 "type String, value 0.23" The complete code follows {-# OPTIONS -fglasgow-exts #-} -- Typechecker to GADT -- Implementing a typed DSL with the typed evaluator and the -- the typechecker from untyped terms to typed ones module Eval where -- Untyped terms (what I get from my parser): data Exp = EDouble Double | EString String | EPrim String | EApp Exp Exp deriving (Show) -- Typed terms: data Term a where Num :: Double -> Term Double Str :: String -> Term String App :: Term (a->b) -> Term a -> Term b Fun :: (a->b) -> Term (a->b) -- Typed evaluator eval :: Term a -> a eval (Num x) = x eval (Str x) = x eval (Fun x ) = x eval (App e1 e2) = (eval e1) (eval e2) -- Types and type comparison data Typ a where TDouble :: Typ Double TString :: Typ String TFun :: Typ a -> Typ b -> Typ (a->b) data EQ a b where Refl :: EQ a a -- checking that two types are the same. If so, give the witness eqt :: Typ a -> Typ b -> Maybe (EQ a b) eqt TDouble TDouble = Just $ Refl eqt TString TString = Just $ Refl eqt (TFun ta1 tb1) (TFun ta2 tb2) = eqt' (eqt ta1 ta2) (eqt tb1 tb2) where eqt' :: (Maybe (EQ ta1 ta2)) -> Maybe (EQ tb1 tb2) -> Maybe (EQ (ta1 -> tb1) (ta2 -> tb2)) eqt' (Just Refl) (Just Refl) = Just Refl eqt' _ _ = Nothing eqt _ _ = Nothing instance Show (Typ a) where show TDouble = "Double" show TString = "String" show (TFun ta tb) = "(" ++ show ta ++ "->" ++ show tb ++ ")" -- Type checking data MostlyStatic = forall t. MostlyStatic (Typ t, Term t) -- Typing environment type Gamma = [(String,MostlyStatic)] -- Initial environment (the types of primitives) env0 = [("rev", MostlyStatic (TFun TString TString, Fun (reverse::String->String))), -- sorry, no polymorphism! ("show", MostlyStatic (TFun TDouble TString, Fun (show::Double->String))), ("inc", MostlyStatic (TFun TDouble TDouble, Fun ((+ (1.0::Double))))), ("+", MostlyStatic (TFun TDouble (TFun TDouble TDouble), Fun ((+)::Double->Double->Double))) ] typecheck :: Gamma -> Exp -> Either String MostlyStatic -- literals typecheck _ (EDouble x) = Right $ MostlyStatic (TDouble, Num x) typecheck _ (EString x) = Right $ MostlyStatic (TString, Str x) typecheck env (EPrim x) = maybe err Right $ lookup x env where err = Left $ "unknown primitive " ++ x typecheck env (EApp e1 e2) = case (typecheck env e1, typecheck env e2) of (Right e1, Right e2) -> typechecka e1 e2 (Left err, Right _) -> Left err (Right _, Left err) -> Left err (Left e1, Left e2) -> Left (e1 ++ " and " ++ e2) -- typecheck the application typechecka (MostlyStatic ((TFun ta tb),e1)) (MostlyStatic (t2,e2)) = typechecka' (eqt ta t2) tb e1 e2 where typechecka' :: Maybe (EQ ta t2) -> Typ tb -> Term (ta->tb) -> Term t2 -> Either String MostlyStatic typechecka' (Just Refl) tb e1 e2 = Right $ MostlyStatic (tb,App e1 e2) typechecka' _ tb e1 e2 = Left $ unwords ["incompatible type of the application:", show (TFun ta tb), "and", show t2] typechecka (MostlyStatic (t1,e1)) _ = Left $ "Trying to apply not-a-function: " ++ show t1 -- tests te1 = EApp (EPrim "inc") (EDouble 10.0) te2 = EApp (EDouble 10.0) (EPrim "inc") te3 = EApp (EApp (EPrim "+") (EApp (EPrim "inc") (EDouble 10.0))) (EApp (EPrim "inc") (EDouble 20.0)) te4 = EApp (EPrim "rev") te3 te5 = EApp (EPrim "rev") (EApp (EPrim "show") te3) -- typecheck-and-eval teval :: Exp -> String teval exp = either (terror) (ev) (typecheck env0 exp) where terror err = "Type error: " ++ err ev (MostlyStatic (t,e)) = "type " ++ show t ++ ", value " ++ (tryshow (eqt t TString) (eqt t TDouble) (eval e)) tryshow :: Maybe (EQ t String) -> Maybe (EQ t Double) -> t -> String tryshow (Just Refl) _ t = t tryshow _ (Just Refl) t = show t tryshow _ _ _ = "<fun>"

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