[Haskell-beginners] how do typeclasses work again?

[Sylvain Henry]

 > I've told it to expect a function "Integer -> String"...surely?

No. The constraints only indicates that an instance matching "Is isx 
Integer" must exist but that's not what the compiler expects.

You have:
(\i -> "") :: x -> String
the type `x` cannot be inferred.

Hence when you write `apply (\i -> "")` the compiler expects an instance 
"Is isx x" for the ambiguous x.

You have to declare the type of `i` to be Integer for your code to work:
foo = apply (\(i :: Integer) -> "")

--
Sylvain


On 09/02/2017 17:59, Nicholls, Mark wrote:
> Sorry..I do haskell about once every 6 months for 2 hours...and then get on with my life.
>
> I always forget some nuance of typeclasses.
>
> Consider some simple typeclass
>
>> class Is isx x where
>>    apply :: (x -> y) -> isx -> y
>
> We can make any type a member of it...mapping to itself
>
>> instance Is x x where
>>    apply f = f
> But we can also make a tuple a member of it...and pull the 1st member..
>
>> instance Is (x,y) x where
>>    apply f (x,y) = f x
> Weird and largey useless...but I'm playing.
>
> Then construct a function to operate on it
>
>> foo2 :: (Is isx Integer) => isx -> String
>> foo2 = apply (\i -> "")
> And...
>
> • Could not deduce (Is isx x0) arising from a use of ‘apply’
>        from the context: Is isx Integer
>          bound by the type signature for:
>                     foo2 :: Is isx Integer => isx -> String
>          at prop.lhs:51:3-43
>        The type variable ‘x0’ is ambiguous
>        Relevant bindings include
>          foo2 :: isx -> String (bound at prop.lhs:52:3)
>        These potential instances exist:
>          instance Is x x -- Defined at prop.lhs:41:12
>          instance Is (x, y) x -- Defined at prop.lhs:45:12
>      • In the expression: apply (\ i -> "")
>        In an equation for ‘foo2’: foo2 = apply (\ i -> "")
>
>
> What's it going on about?
> (my brain is locked in F# OO type mode)
>
> I've told it to expect a function "Integer -> String"...surely?
> Whats the problem.
>
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