[Haskell-beginners] Return a Foldable instance

[Brandon S Allbery KF8NH]

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On 5/3/11 21:31 , Federico Mastellone wrote:
> 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]
> 
> 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, ....]

Right; you're not returning a call-point-specified Foldable, you're
returning a list always.  I suspect you really want Traversable, not
Foldable, so you can map elements from list to arbitrary Traversable; the
reverse is then Data.Foldable.toList.  (All Traversables are Foldables.)

http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.3.1.0/Data-Traversable.html

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]    allbery.b at gmail.com
system administrator  [openafs,heimdal,too many hats]                kf8nh
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