- Haskell-Cafe - Haskell.org

runProcess does not care about hSetBuffering?
by Magicloud Magiclouds 29 Jul '09

29 Jul '09

Hi, Code like: (or_, ow_) <- createPipe or <- fdToHandle or_ ow <- fdToHandle ow_ hSetBuffering ow LineBuffering hSetBuffering or LineBuffering h <- runProcess cmd [] Nothing Nothing Nothing (Just ow) Nothing In the cmd process, the ow is not LineBuffering.... -- 竹密岂妨流水过山高哪阻野云飞

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an instance in other than the last type parameters
by Petr Pudlak 28 Jul '09

28 Jul '09

Hi, I have probably a very simple question, but I wasn't able to figure it out myself. Consider a two-parameter data type: > data X a b = X a b If I want to make it a functor in the last type variable (b), I can just define > instance Functor (X a) where > fmap f (X a b) = X a (f b) But how do I write it if I want X to be a functor in its first type variable? Is that possible at all? Something like: > instance Functor ??? where > fmap f (X a b) = X (f a) b Thanks in advance, Petr

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ghci identifier listing question
by Daniel van den Eijkel 28 Jul '09

28 Jul '09

Hi, in GHCi, after pressing the tab-key, all defined identifiers are listed. If an identifier was redefined, all old and shadowed versions are listed, too. Is it possible, to reach the (shadowed) values in any way? Another question: Is it possible to show only those identifiers that are defined during the interactive session? I only can list all identifiers which start with a given prefix, but I would like to know if GHCi can show me all identifiers defined in a given module or in the current session (by pressing Tab or alike). Best regards, Daniel

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A Question of Restriction
by Brian Troutwine 28 Jul '09

28 Jul '09

Hello all. I would like to define a data type that is the super-set of several types and then each of the proper subset types. For example: data Foo = One | Two | Three | Four data Odd = One | Three data Even = Two | Four This, of course, does not work. It seems that such a thing should possible to express entirely in the type system, but I cannot think of how. Would someone be so kind as to explain how this sort of thing can be accomplished? Thanks, Brian

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Importing Control.Arrow changes inferred type of (m >>= f) x in ghci
by Dan Weston 28 Jul '09

28 Jul '09

The following inferred type has a constraint that can be trivially satisfied, but isn't: Control.Monad> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (Monad ((->) t)) => (t -> a, a -> t -> b, t) -> b -- In Control.Monad there is forall t. instance Monad ((->) t), -- so why is the vacuous Monad constraint still there? -- Nor can I remove it with a type annotation: Control.Monad> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] -- Then if I just import a module: Control.Monad> :m + Control.Arrow -- Now, the Monad ((-> t) constraint disappears: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b -- Although it still will not accept an explicit type annotation: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] Is there some explicit kind annotation I can/should give to get the most general type? Dan

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Re: [Haskell] Re: 20 years ago
by Ketil Malde 28 Jul '09

28 Jul '09

[redirected from haskell@] Benjamin L.Russell <DekuDekuplex(a)Yahoo.com> writes: >> One often amusing outgrowth of this is that FP (OOP) fanatics anthropomorphize >> their functions (objects). Well, I don't think we do. Functions are just mappings of values to values, they may be opaque, but they're predictable, unchanging, and just...too boring to be antropomorphized. Objects contain all kinds of hidden state and dependencies, and the sheer unpredicatability of it all is the reason for the anthropomorphics - it a symptom of a disease, not a desirable quality. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I you use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

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Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

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'cabal install lhs2tex' fails for me on os x -- help please
by Conal Elliott 27 Jul '09

27 Jul '09

Fails during configuration: bash-3.2$ cabal install lhs2tex Resolving dependencies... [1 of 1] Compiling Main ( /tmp/lhs2tex-1.1423397/lhs2tex-1.14/Setup.hs, /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/Main.o ) Linking /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/setup ... Configuring lhs2tex-1.14... cabal: Error: some packages failed to install: lhs2tex-1.14 failed during the configure step. The exception was: exit: ExitFailure 11 Note that the the first module to be compiled is Main. On my linux machine, Main is the *last* of several modules to be compiled. Here's my failure log: http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3096#a3096 . If instead I use ./configure and make, i get errors due to use of the old Exception API: ... [17 of 19] Compiling FileNameUtils ( FileNameUtils.lhs, FileNameUtils.o ) FileNameUtils.lhs:81:35: Ambiguous type variable `b' in the constraint: `GHC.Exception.Exception b' arising from a use of `catch' at FileNameUtils.lhs:(81,35)-(89,60) Probable fix: add a type signature that fixes these type variable(s) ... Suggestions greatly appreciated! - Conal

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Retiring forkIO
by Günther Schmidt 27 Jul '09

27 Jul '09

Hi, I'd like to retire forkIO and friends by using Delimited Continuations instead. Am I dead wrong here in my understanding of Delimited Continuations or can I pursue in this direction? The most immediate use for this is actully GUI problems where I'd use del-conts in place of forkIO for long-running computations because once I forkIO the computation so as not to block the UI thread I cannot call GUI code from the end of that thread. Well I can but then the app crashes because of single threading issues. Günther

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