Unknown type variable
This error always refers to a type definition, which uses a type variable that was not a parameter of the type being defined. For example:
Disco> type T(a,b) = N * c
Error: Unknown type variable 'c'.
In this example, we are defining the type T which has parameters
a and b. We are thus allowed to use a and b anywhere
inside the definition of T. However, here we use c, which is
not defined.
Did you misspell a variable name?
Did you forget to add the variable as a parameter of the type? For example, if we want to define a type of parameterized trees, but write
type T = Unit + a * T * T, we would get this error; what we should write insitead istype T(a) = Unit + a * T(a) * T(a).