[Haskell-beginners] map type explanation

[Lawrence Bottorff]

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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