-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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-Trav... - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery.b@gmail.com system administrator [openafs,heimdal,too many hats] kf8nh -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk3AvUkACgkQIn7hlCsL25UVCACcCVHdZL8u2JwD0WZ0P4V6+u3+ XcQAoLI/mqsekYCA2SmlyAfMNpn5Kjem =I0YQ -----END PGP SIGNATURE-----