March 2010 - Glasgow-haskell-users

(read getLine) + (read getLine)
by Tyson Whitehead 04 Mar '10

04 Mar '10

Consider what would happen if GHC optionally inserted a morphism on fexps (the expression at the head of an application) as it currently does with integer literals and optionally with string literals. A standard function call f x y would windup as morph (morph f x) y With the following basic classes -- Functor class F t where func :: (a -> b) -> t a -> t b -- Pointed class F t => P t where -- pure :: a -> t a -- Applicative class P t => A t where apply :: t (a -> b) -> t a -> t b -- Monad class A t => M t where join :: t (t a) -> t a we can define a standard set of morphisms -- Category class C a b where morph :: a -> b instance C (a -> b) (a -> b) where morph = id instance F t => C (a -> b) (t a -> t b) where morph = func instance P t => C (a -> b) (a -> t b) where morph f = func f . pure instance A t => C (t (a -> b)) (t a -> t b) where morph = apply instance A t => C (t (a -> b)) (a -> t b) where morph f = apply f . pure instance M t => C (t (a -> t b)) (t a -> t b) where morph f = join . apply f instance M t => C (t (a -> t b)) (a -> t b) where morph f = join . apply f . pure instance M t => C (a -> t b) (t a -> t b) where morph f = join . apply (pure f) instance M t => C (a -> t b) (a -> t b) where morph f = join . apply (pure f) . pure If GHC could correctly pick between them (possibly if just tried them from most specific to least?), then we would get the following transformation (read getLine) + (read getLine) --> morph (morph (+) (morph read getLine)) (morph read getLine) --> apply (func (+) (func read getLine)) (func read getLine) or, using the names in the standard library (include Control.Applicative), (<*>) (fmap (+) (fmap read getLine)) (fmap read getLine) which does the right thing. Anyway, thought I would throw it out there in case it could be made to work. Cheers! -Tyson

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Issue with type families
by Tyson Whitehead 04 Mar '10

04 Mar '10

The following code {-# LANGUAGE FlexibleInstances, TypeFamilies #-} import Control.Applicative class Z t where type W t z :: t -> W t instance Z (a -> b) where type W (a -> b) = a -> b z = id instance Z (IO (a -> b)) where type W (IO (a -> b)) = IO a -> IO b z = (<*>) works fine, but if I try and generalize to from IO to the Applicative classes instance (Applicative m) => Z (m (a -> b)) where type W (m (a -> b)) = m a -> m b z = (<*>) I get the following error Temp.hs:10:9: Conflicting family instance declarations: type instance W (a -> b) -- Defined at Temp.hs:10:9 type instance W (m (a -> b)) -- Defined at Temp.hs:14:9 Failed, modules loaded: none. unless I remove one of the instances, and then it is happy. Is this correct? I don't claim to really understand the rules regarding type classes, but I can't see why these are overlapping. Thanks! -Tyson

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stg-ap-1-upd-info
by Herk, Robert van 03 Mar '10

03 Mar '10

Hi All, I am optimizing a program to consume less memory. If I profile it with +RTS -hd, it reports to fill 50% of my memory with stg-ap-1-upd-info. Does this mean anything? Regards, Robert The information contained in this message may be confidential and legally protected under applicable law. The message is intended solely for the addressee(s). If you are not the intended recipient, you are hereby notified that any use, forwarding, dissemination, or reproduction of this message is strictly prohibited and may be unlawful. If you are not the intended recipient, please contact the sender by return e-mail and destroy all copies of the original message.

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I accidentally the Prelude
by Josef Svenningsson 03 Mar '10

03 Mar '10

Hi, It seems I've been able to mess up my ghc installation pretty badly. Here is what happens if I just try to invoke ghci from the prompt: $ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <command line>: module `Prelude' is not loaded $ I have no idea what I did to end up in this situation. What I've been doing lately is reinstalling some packages. I also have another ghc installed but it's at a completely different place in the file system. The only thing I can think of is if cabal managed to somehow confuse the two ghcs and wrote some data in the wrong place. What I really would like to know is if there is a simple way to fix this without completely reinstalling ghc with all the libraries I have installed. Has anyone else experienced anything similar? If this is a potential bug I'd be happy to provide any data that might help track it down. Cheers, Josef

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Re: Easily generating efficient instances for classes
by John Lato 02 Mar '10

02 Mar '10

> From: Christian H?ner zu Siederdissen > > Hi, > > I am thinking about how to easily generate instances for a class. Each > instance is a tuple with 1 or more elements. In addition there is a > second tuple with the same number of elements but different type. This > means getting longer and longer chains of something like (...,x3*x2,x2,0). > > - template haskell? > - CPP and macros? > > Consider arrays with fast access like Data.Vector, but with higher > dimensionality. Basically, I want (!) to fuse when used in Data.Vector > code. (shameless plug) You may want to look at my AdaptiveTuple package, which does something very similar to this. I used Template Haskell because AFAIK neither generic approaches nor DrIFT/Derive will generate data decls. If all you need are the instances, then DrIFT or Derive would be my recommendations. Cheers, John

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C that setjmps -> haskell -> C that might longjmp
by Tyson Whitehead 02 Mar '10

02 Mar '10

If I have the following call sequence C code -> Haskell code -> various C code bits where the various C code bits on the right might do a longjmp (their version of an exception) and jumping back to the C code on the left. Is it possible to have C code on the left then somehow tell GHC to cleanup the aborted Haskell code/resume executing it with an exception, or is the only option to setjmp wrap all the various C code bits on the right? Thanks! -Tyson

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