[Haskell-cafe] constant functions
michael rice
nowgate at yahoo.com
Wed Dec 27 22:55:11 EST 2006
Thanks! I figured I was close.
Didn't even know const was available.
I put together a compliment functions earlier
complement :: (a -> Bool) -> a -> Bool
complement p x = not (p x)
By the signature, the first argument is a function
(predicate) which when given a value returns a Bool?
And the second argument is just a value? And the
function returns a Bool?
> map (complement odd) [1,2,3,4,5,6]
[False,True,False,True,False,True]
>
By similar reasoning the always function would seem to
have a signature
a -> (b -> a)
where the first argument is just a value and the
return value is a function that when given a possibly
different value just returns the value originally
given to always?
Is that reasoning OK? Are
a -> (b -> a) and a -> b -> a the same signature?
So the inferred type is usually pretty accurate? These
signatures are a bit confusing. Is there a good
tutorial?
I'm using Hugs/Win XP just to scope out the language
right now. I tried what you suggested and got
Hugs> let always x _ = x
ERROR - Syntax error in expression (unexpected end of
input)
Hugs>
Isn't Hugs an interpreter?
Thanks again. Really interesting language Haskell.
Michael
--- Matthew Brecknell <haskell at brecknell.org> wrote:
> > This is what I've been trying:
> >
> > always :: (a -> a) -> a -> a
> > always x = (\y -> x)
>
> Your function implementation is correct, but the
> type is wrong. Try
> this:
>
> always :: a -> b -> a
>
> Or, just use the function "const", from the Prelude.
> :-)
>
> The type system can be very handy when learning
> Haskell. If you think
> you have the correct implementation but can't work
> out the type, just
> start up an interpreter and ask it for the inferred
> type. For example:
>
> Prelude> let always x _ = x
> Prelude> :t always
> always :: t -> t1 -> t
>
> Once you have the type, ask Hoogle if the function
> already exists:
>
> http://haskell.org/hoogle/?q=t+-%3E+t1+-%3E+t
>
> And there is "const" at the top of the results. :-)
>
>
>
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