Question

I want to place signature/structure pair inside a structure, like so:

structure Outer :> OUTER =
struct
    signature INNER =
    sig
        ...
    end

    structure Inner :> INNER =
    struct
    ...
    end
end

but even the simplest of examples produces an error:

../test.sml:1.18-2.6 Error: syntax error: replacing  STRUCT with  EQUALOP
../test.sml:5.6 Error: syntax error found at END

It appears that signatures are not allowed inside structures. What is the best way to achieve this functionality?

Answer 1

Although structures nest in SML, signatures do not. It’s not clear what functionality you want to achieve:

• Hiding a named signature is impossible.

• Having the INNER signature depend on types declared in structure Outer is achieved through fibration (the where type clause). There’s a length section on fibration in the tutorial by Harper and Pierce in Benjamin Pierce’s book on advanced types in programming languages.