[Haskell-beginners] Return a Foldable instance

[Antoine Latter]

On Tue, May 3, 2011 at 8:31 PM, Federico Mastellone <fmaste at gmail.com> wrote:
> Hi,
>
> I want to make a function that returns a Foldable instance, briefly,
> something like this:
>
> import Data.Foldable
>
> test :: Foldable f => f Int
> test = [1,2,3,4]
>

One point that helped me figure things out:

The signature:

> test :: Foldable f => f Int

does NOT mean "the value test satisfies the 'Foldable' contract'

It means "the value test is of ANY 'Foldable' type". As in, it is of
whatever type the caller picks, so long as it is 'Foldable'. And a
list is not 'whatever type the caller picks.'

Type classes are used in Haskell to write polymorphic functions, not
for implementation hiding:

> myNumericalCalculation :: Fractional n => n -> n -> Bool -> n

The caller can pick if they want to use limited precision Floats or
Doubles, or if they want to pay the price for infinite precision Ratio
types, or some type that I hadn't even thought of when I wrote the
functions.

Antoine

> But I get this error:
>
>    Couldn't match expected type `f' against inferred type `[]'
>        `f' is a rigid type variable bound by
>            the type signature for `test' at Test.hs:3:17
>    In the expression: [1, 2, 3, 4]
>    In the definition of `test': test = [1, 2, 3, ....]
>
> How can I make a function like the one above?
>
> I'm creating different implementations of a multimap, using a Set and
> a [], and instead of providing functions getValuesList and
> getValuesSet that return a [] and a Set respectively I want to provide
> a more generic one, getValues that returns a Foldable instance.
>
> Thanks!!!
>
> --
> Federico Mastellone
> Computer Science Engineer - ITBA
> ".. there are two ways of constructing a software design: One way is
> to make it so simple that there are obviously no deficiencies, and the
> other way is to make it so complicated that there are no obvious
> deficiencies. The first method is far more difficult."
>
> Tony Hoare, 1980 ACM Turing Award Lecture.
>
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