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Today's Topics:
1. Syntax error using where with guards. (Manuel Vázquez Acosta)
2. Re: Syntax error using where with guards. (Francesco Ariis)
3. Re: Syntax error using where with guards. (mva....@gmail.com)
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Message: 1
Date: Fri, 15 Sep 2017 18:53:54 -0400
From: Manuel Vázquez Acosta <mva....@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Syntax error using where with guards.
Message-ID: <87r2v74lq5....@pavla.com>
Content-Type: text/plain; charset=utf-8
I was trying to implement a functional version of numerator/denominator
simplifications of symbols.
I came up with:
simplify :: (Eq o, Ord o) => [o] -> [o] -> ([o], [o])
simplify [] bs = ([], bs)
simplify ts [] = (ts, [])
simplify (x:xs) (y:ys)
| x == y = simplify xs ys
| x < y =
([x] ++ (fst $ simplify xs (y:ys)), snd $ simplify xs (y:ys))
| otherwise =
(fst r, [y] ++ snd r) where r = simplify (x:xs) ys
simplify2 :: (Eq o, Ord o) => [o] -> [o] -> ([o], [o])
simplify2 a b = simplify (sort a) (sort b)
-- sort not shown here
In the second guard of the simplify function the expression 'simplify xs
(y:ys)' is repeated. I would like to write instead:
simplify (x:xs) (y:ys)
| x == y = simplify xs ys
| x < y =
([x] ++ fst r, snd r) where r = simplify xs (y:ys)
| otherwise =
(fst r, [y] ++ snd r) where r = simplify (x:xs) ys
But doing so is parse error:
$ ghc xoutil/dim/meta.hs
[1 of 1] Compiling Meta ( xoutil/dim/meta.hs, xoutil/dim/meta.o
)
xoutil/dim/meta.hs:19:3: parse error on input ‘|’
I'm using: ghc 7.10.3.
Any ideas of why this is a syntax error?
Best regards,
Manuel.
------------------------------
Message: 2
Date: Sat, 16 Sep 2017 01:49:00 +0200
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Syntax error using where with guards.
Message-ID: <20170915234900.4iaz6q2umv7dx...@x60s.casa>
Content-Type: text/plain; charset=utf-8
On Fri, Sep 15, 2017 at 06:53:54PM -0400, Manuel Vázquez Acosta wrote:
> $ ghc xoutil/dim/meta.hs
> [1 of 1] Compiling Meta ( xoutil/dim/meta.hs,
> xoutil/dim/meta.o )
>
> xoutil/dim/meta.hs:19:3: parse error on input ‘|’
Hello Manuel,
`where` scopes over several guarded equations (and this is the main
difference with `let`). This should do the trick:
| a == 1 = let x = 10
in x + a
| a == 2 = let x = 3
in x + a
------------------------------
Message: 3
Date: Fri, 15 Sep 2017 21:37:58 -0400
From: mva....@gmail.com
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Syntax error using where with guards.
Message-ID: <87lglf4e4p....@pavla.com>
Content-Type: text/plain; charset=utf-8
Thanks, that solves it. I really like `where`, though.
Francesco Ariis <fa...@ariis.it> writes:
> On Fri, Sep 15, 2017 at 06:53:54PM -0400, Manuel Vázquez Acosta wrote:
>> $ ghc xoutil/dim/meta.hs
>> [1 of 1] Compiling Meta ( xoutil/dim/meta.hs,
>> xoutil/dim/meta.o )
>>
>> xoutil/dim/meta.hs:19:3: parse error on input ‘|’
>
> Hello Manuel,
> `where` scopes over several guarded equations (and this is the main
> difference with `let`). This should do the trick:
>
> | a == 1 = let x = 10
> in x + a
> | a == 2 = let x = 3
> in x + a
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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