How to deal with Agda not being sure in whether to generate constructor case in the `with` statement?

Question

I have the following code:

open import Data.Nat
open import Agda.Builtin.Char
open import Data.Maybe

digit' : ℕ → Maybe ℕ
digit' n with compare n (primCharToNat '9')
... | greater _ _ = nothing
... | _ = ?

digit : Char → Maybe ℕ
digit c = digit' (primCharToNat c)

Unfortunately, Agda "load file" command in emacs fails with the following message:

tmp.agda:7,1-8,12
I'm not sure if there should be a case for the constructor less,
because I get stuck when trying to solve the following unification
problems (inferred index ≟ expected index):
  m ≟ n
  suc (m + k) ≟ 57
when checking the definition of with-6

As far as I understand, Agda understands that primCharToNat '9' is a constant and is not sure whether there are any preconditions on n being always less than primCharToNat '9' (57). Therefore, it is not sure whether it should generate the less case (I assume it's also the case with equal case). Can I somehow force Agda into generating all cases?

Background: I'm trying to write a digit : Char → Maybe ℕ function which should return either just x if the character passed is a decimal digit or nothing if it's not. I think about the following algorithm: convert the argument to a natural number (ASCII code, probably) with primCharToNat and then compare it with primCharToNat '0' and primCharToNat '9'.

Answer 1

A possible solution consists in abstracting over primCharToNat '9':

digit' : ℕ → Maybe ℕ
digit' n with primCharToNat '9' | compare n (primCharToNat '9')
... | _ | greater _ _ = nothing
... | _ | _ = {!!}