(kernel) unknown constant

Stream: lean4

[Sébastien Michelland]

I have this error, which looks like a Lean bug. It seems that the double-nested dependency is required, the class parameter (specifically a class) is required as well, and the mutual recursion in the string functions is also needed.

class Class where
instance empty: Class := {}

mutual
inductive T (δ: Class) where
| mk (ss: List (S δ))

inductive S (δ: Class) where
| mk (ts: List (T δ))
end

mutual
partial def str_T: T δ → String
| .mk ss => "\n".intercalate (ss.map str_S)

partial def str_S: S δ → String
| .mk ts => "\n".intercalate (ts.map str_T)
end

def error := str_T (T.mk (δ := empty) [])
-- (kernel) unknown constant 'str_T._at.error._spec_1'

Cc @Siddharth Bhat

[Sébastien Michelland]

Temporary workaround for us: make the str_* functions total by not using map.

[Leonardo de Moura]

Pushed a fix for this.