[Haskell-beginners] map type explanation
Ut Primum
utprimum at gmail.com
Fri Dec 18 22:08:56 UTC 2020
Hi,
a -> b is the type of a function taking arguments of a generic type (we
call it a) and returning results of another type, that we call b.
So
(a -> b ) -> [a] -> [b]
Means that you have a first argument that is a function (a-> b), a second
argument that is a list of elements of the same type of the function input,
and that the returned element is a list of things of the type of the output
of the function.
Cheers,
Ut
Il ven 18 dic 2020, 23:02 Lawrence Bottorff <borgauf at gmail.com> ha scritto:
> Thank you, but why in
>
> map :: (a -> b) -> [a] -> [b]
>
> are there parentheses around a -> b ? In general, what is the currying
> aspect of this?
>
>
> On Fri, Dec 18, 2020 at 12:43 PM David McBride <toad3k at gmail.com> wrote:
>
>> They are not parameters, they are the types of the parameters.
>>
>> In this case a can really be anything, Int, Char, whatever, so long as
>> the function takes a single argument of that type and the list that is
>> given has elements of that same type.
>> It is the same for b, it doesn't matter what b ends up being, so long as
>> when you call that function the function's return value is compatible with
>> the element type of the list that you intended to return from the entire
>> statement.
>>
>> You can mess with it yourself in ghci to see how type inference works.
>>
>> >:t show
>> :show :: Show a => a -> String
>> >:t map show
>> map show :: Show a => [a] -> [String]
>> > :t flip map [1::Int]
>> > flip map [1::Int] :: (Int -> b) -> [b]
>>
>>
>> On Fri, Dec 18, 2020 at 1:31 PM Lawrence Bottorff <borgauf at gmail.com>
>> wrote:
>>
>>> I'm looking at this
>>>
>>> ghci> :type map
>>> map :: (a -> b) -> [a] -> [b]
>>>
>>> and wondering what the (a -> b) part is about. map takes a function and
>>> applies it to an incoming list. Good. Understood. I'm guessing that the
>>> whole Haskell type declaration idea is based on currying, and I do
>>> understand how the (a -> b) part "takes" an incoming list, [a] and
>>> produces the [b] output. Also, I don't understand a and b very well
>>> either. Typically, a is just a generic variable, then b is another
>>> generic variable not necessarily the same as a. But how are they being
>>> used in this type declaration?
>>>
>>> LB
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