Attempt to improve function under-application and over-application errors - #5665

Overview

Uses heuristics to detect function over and under application and provide better information on how to fix it.

Under-applied:

main : 'Nat
main = do
  multiply x y = x Nat.* y
  add x y = x Nat.+ y
  doMath x y = add (multiply x y)
  doMath 3 4

Before:

After:

And a case where a function is over-applied.

main2 : 'Nat
main2 = do
  multiply x y = x Nat.* y
  add x y = x Nat.+ y
  doMath x y = add (multiply x y) 2 3
  doMath 3 4

Before

After

Implementation notes

• Pretty small tweak to the typechecker which checks function arities on mismatched types and specializes the selected error message.
• New arity function which ignores effects for this purpose
• New error message formatting
• new transcripts

Interesting/controversial decisions

There may be cases where this is incorrect, but I think the benefit is worth that risk, we can dogfood it and tweak or roll-back as needed.

Test coverage

See new transcripts