[Haskell-beginners] applicative default structure

David McBride toad3k at gmail.com
Thu Dec 22 15:33:41 UTC 2016


Rather than bailing with an instance error for both Applicative and Num,
which is the technically the right way, and the way that it used to be in
the dark ages of ghci.  Instead it chooses types which are probably what
you wanted.  In this case it defaults f to IO and a to Int, and then runs
it.

You can read more about type defaulting in ghci here:
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html

On Thu, Dec 22, 2016 at 10:18 AM, Ovidiu Deac <ovidiudeac at gmail.com> wrote:

> My understanding is that x can take any form required for type-inference.
> That's fine but what is the "default" structure if you don't specify any?
>
> On Thu, Dec 22, 2016 at 5:13 PM, Imants Cekusins <imantc at gmail.com> wrote:
>
>> > What is f here?
>>
>> anything Applicative:
>>
>> Prelude> let a1 = pure 1
>> Prelude> let a2 = pure 1
>>
>> Prelude> (a1::Maybe Int) == a2
>> True
>> Prelude> (a1::Maybe Float) == a2
>> True
>> Prelude> (a1::Either String Float) == a2
>> True
>>
>>
>> ‚Äč
>>
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>>
>>
>
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