Strange behavior with a mapping-function

[mat...@flow.net]

Hey Ambrose,

The following is giving me trouble:

(defalias Pair '[String String])

(ann get-second-fails [(t/Vec Pair) -> (t/Seqable String)])

(defn get-second-fails [in]

  (map second in))

(ann get-second-works [(t/Vec Pair) -> (t/Seqable String)])

(defn get-second-works [in]

  (map (t/fn [i :- Pair] (second i)) in))

In a nutshell, passing the second function directly to map doesn't work - I have to wrap it in the core.typed function macro.

What's up with that?

Cheers.

(error is:

Type Error (rest_api/types.clj:459:3) Polymorphic function map could not be applied to arguments:

Polymorphic Variables:

c

a

b

Domains:

[a b ... b -> c] (t/NonEmptySeqable a) (t/NonEmptySeqable b) ... b

[a b ... b -> c] (t/U (Seqable a) nil) (t/U (Seqable b) nil) ... b

Arguments:

(t/All [x] (t/IFn [(t/HSequential [t/Any x t/Any *]) -> x :object {:id 0, :path [(Nth 1)]}] [(t/Option (t/I (t/CountRange 0 1) (Seqable x))) -> nil] [(t/I (t/CountRange 2) (Seqable x)) -> x] [(t/Option (Seqable x)) -> (t/Option x)])) (IPersistentVector Pair)

Ranges:

(t/NonEmptyASeq c)

(t/ASeq c)

with expected type:

(t/Seqable String)

in: (map second in)

[Ambrose Bonnaire-Sergeant]

Hi Matthew,

Polymorphic arguments to polymorphic functions must be instantiated. See this blog.

Something like (map (inst second String) in) should work.

Thanks,

Ambrose