[Haskell-beginners] Understanding the type signature of flip$id
Christian Maeder
Christian.Maeder at dfki.de
Wed Mar 2 08:51:43 CET 2011
Maybe "flip ($)" is clearer? ($) is identity over function types.
If you have the (first) argument you could use a section like "($ a)"
and avoid flip.
C.
Am 02.03.2011 06:33, schrieb Jeff Lasslett:
> Thanks Daniel,
>
> I think I follow what you've written.
>
> (b -> c) -> (b -> c) is the same as (b -> c) -> b > c
>
> and when that is flipped: b -> (b- > c) -> c
>
> Is that right?
>
> Thanks,
> Jeff
>
>
>
> On 2 March 2011 11:23, Daniel Fischer <daniel.is.fischer at googlemail.com> wrote:
>> On Wednesday 02 March 2011 00:43:23, Jeff Lasslett wrote:
>>> I just don't understand how passing id to flip results in the
>>> following type signature:
>>>
>>> Prelude> :t flip$id
>>> flip$id :: b -> (b -> c) -> c
>>>
>>> I do understand what flip has done to map here:-
>>>
>>> Prelude> :t flip$map
>>> flip$map :: [a] -> (a -> b) -> [b]
>>>
>>> map take a function and a list and produces a new list. If map is
>>> passed to flip the result is a function that takes a list, then a
>>> function and results in a new list.
>>>
>>> How do we go from flip having this signature:
>>>
>>> Prelude> :t flip
>>> flip :: (a -> b -> c) -> b -> a -> c
>>> Prelude>
>>>
>>> and id having
>>>
>>> Prelude> :t id
>>> id :: a -> a
>>> Prelude>
>>>
>>> to flip$id looking like flip$id :: b -> (b -> c) -> c ???
>>>
>>> Thanks,
>>> Jeff
>>
>> The point is that the type of id has to be unified with the type of flip's
>> (first) argument.
>>
>> flip :: (a -> b -> c) -> (b -> a -> c)
>> id :: t -> t
>>
>> So we have to unify (a -> b -> c) and (t -> t). Fully parenthesized,
>> a -> b -> c is a -> (b -> c). Now unification yields
>>
>> t = a
>> -- id's arg must have the same type as the flip's argument's arg
>>
>> and
>>
>> t = (b -> c)
>> -- id's result must have the same result as flip's argument's result
>>
>> From that follows a = (b -> c) and *id can be passed to flip only at a more
>> restricted type than id's most general type, namely at the type
>> id :: (b -> c) -> (b -> c)*
>>
>> So,
>>
>> flip (id :: (b -> c) -> b -> c) :: b -> (b -> c) -> c
>>
>
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