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Today's Topics:
1. Re: partial application (Peter Hall)
----------------------------------------------------------------------
Message: 1
Date: Wed, 5 Dec 2012 02:47:31 +0000
From: Peter Hall <[email protected]>
Subject: Re: [Haskell-beginners] partial application
To: Brent Yorgey <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<CAA6hAk4d-PxXv=cf1sz8bnhrzsvwm_y+nnhqd8cupqof0ds...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
> >
> > g = (\x -> f x 1 2 3)
>
> Yes, a lambda is the only way to do it.
This is not exactly the same thing, but if you have a function that accepts
only two arguments, you can use `flip` to partially apply with the other
one:
f a b = a ++ b
g = (flip f) "xyz"
g "abc" -- "abcxyz"
To extend that to functions with more parameters, you'd have to create an
equivalent of `flip` for each arity, and the desired permutations might be
more complicated than just reversing them. Still, in the two-argument case,
flip can often be cleaner than a lambda.
Peter
On 3 December 2012 12:53, Brent Yorgey <[email protected]> wrote:
> On Mon, Dec 03, 2012 at 12:28:08PM +0000, Miguel Negrao wrote:
> > Hi list,
> >
> > Is there a syntax in Haskell for partial application that would be
> something like this the following ?
> >
> > f a b c d = a +b+ c+ d
> >
> > g = f _ 1 2 3
> >
> > or the only way to do it is like below ?
> >
> > g = (\x -> f x 1 2 3)
>
> Yes, a lambda is the only way to do it.
>
> > Also, hlint complains about top-level functions without type even
> > if they are not exported out of the corresponding module. Is is
> > really bad style not put type signatures in all top-level functions
> > ? I really like the fact that haskell takes care of the type
> > signatures for me.
>
> Yes, it is bad style. Let me give you two reasons why I always
> encourage putting type signatures on all top-level functions. Whether
> they are exported or not really makes no difference.
>
> 1. Writing the type signature for a function *first*, before
> implementing it, really helps a LOT in clarifying things in your
> own mind. If you cannot write down the intended type of a
> function then you do not understand what it is supposed to do.
> If you do not understand what it is supposed to do then how do
> you expect to be able to implement it?
>
> 2. Suppose you make a mistake when implementing a function. If you
> don't give a type signature, it's possible that GHC will infer
> some type for it anyway (but not the type you intended!). Now
> you will not get an error until further down the line when you
> use that function somewhere else. And what's more, the error
> will not tell you anything about the real problem. It will just
> say "X does not match Y on line 297" and you will have to do a
> lot of work to figure out that the real problem is that you
> messed up in implementing function foo on line 43. But if you
> had put a type signature on foo in the first place, you would
> have gotten an error immediately.
>
> -Brent
>
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