Can Idris infer indices in types of top-level constants?

Question

For example, Agda allows me to write this:

open import Data.Vec
open import Data.Nat

myVec : Vec ℕ _
myVec = 0 ∷ 1 ∷ 2 ∷ 3 ∷ []

and myVec will have type Vec ℕ 4 as expected.

But if I try the same in Idris:

import Data.Vect

myVec : Vect _ Nat
myVec = [0, 1, 2, 3]

I get an error message from the typechecker:

 When checking right hand side of myVec with expected type
         Vect len Nat

 Type mismatch between
         Vect 4 Nat (Type of [0, 1, 2, 3])
 and
         Vect len Nat (Expected type)

 Specifically:
         Type mismatch between
                 4
         and
                 len

Is there a way to define myVec in Idris without manually specifying the index of the Vect?

Answer 1

Per the comments, top-level holes in Idris was universally quantified, instead of being filled via term inference.

I believe (but, ultimately someone from the development team would have to confirm / deny) that this was done partially to encourage explicit types, and thus type-directed development, and partially to have a nice syntax for don't-care values in interface implementations like:

Uninhabited v => Uninhabited (_, v) where
    uninhabited (_, void) = uninhabited void

This don't-care use of the underscore is adopted from it's use in patterns, rather than it's use in expressions.

---

For something like this (it's not exactly what you want, but it is robust against changes to the constant), you can use an explicit existential:

fst : DPair t _ -> t
fst (x ** _) = x

snd : (s : DPair _ p) -> p (fst s)
snd (_ ** prf) = prf

myVecEx : (n ** Vect n Nat)
myVecEx = (_ ** [0, 1, 2, 3])

myVec : Vect (fst myVecEx) Nat
myVec = snd myVecEx

fst and snd might be in the standard library under different names, but I didn't find then in a quick search.

EDIT: An upvote recently drew this answer to my attention again. If you are using Idris 2, I believe you can use a ? instead of a _ at the top level to have it filled in my idris. _ at the top level, is an erased, implicit, unnamed parameter, still. https://idris2.readthedocs.io/en/latest/implementation/overview.html#additional-type-inference