[Haskell-cafe] Re: Suggestions for improvement

[N. Raghavendra]

At 2010-10-03T22:45:30+02:00, Dominique Devriese wrote:

> You need a function like the following for that:
>
> comma :: (a -> b) -> (a -> c) -> a -> (b,c)
> comma f g x = (f x, g x)
>
> Then you could say:
>
> blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength
>
> Ignore this if you haven't read about Applicative or type classes yet,
> but using the Applicative instance for arrow types (->) a, you can
> also write
>
> comma = liftA2 (,)
>
> or
>
> blowup = (uncurry (++)) . liftA2 (,) (blowup . allButLast) lastToTheLength

I tried both of them, but they don't seem to work:

    -- Pointfree blowup.
    blowup1 :: String -> String
    blowup1 = (uncurry (++)) . comma1 (blowup1 . allButLast) lastToTheLength
    
    comma1 :: (a -> b) -> (a -> c) -> a -> (b,c)
    comma1 f g x = (f x, g x)
    
    blowup2 :: String -> String
    blowup2 = (uncurry (++)) . comma2 (blowup2 . allButLast) lastToTheLength

    -- Imported Control.Applicative earlier.
    comma2 :: (a -> b) -> (a -> c) -> a -> (b,c)
    comma2 = liftA2 (,)

% ghci
GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> :l Chapter01.hs 
[1 of 1] Compiling Chapter01        ( Chapter01.hs, interpreted )
Ok, modules loaded: Chapter01.
*Chapter01> comma1 allButLast lastToTheLength "abcd"
("abc","dddd")
*Chapter01> comma2 allButLast lastToTheLength "abcd"
("abc","dddd")
*Chapter01> blowup1 "abcd"
"^CInterrupted.
*Chapter01> blowup2 "abcd"
"^CInterrupted.

It looks like both the above versions of blowup go into some infinite
recursion, and have to be interrupted.

Regards,
Raghavendra.

-- 
N. Raghavendra <raghu at mri.ernet.in> | http://www.retrotexts.net/
Harish-Chandra Research Institute   | http://www.mri.ernet.in/
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