1.7: Module Signatures

In Standard ML, modules expose all of their contents by default – you likely noticed how noisy the Module signature was before, what with it containing all of the contents of opening String – but what is exported may be controlled by defining a signature of the exports for the module.

(* REPL *)
- use "examples/mod-sigs.sml";
structure Math : sig val e : real end

- Math.e;
- val it = 2.7182 : real
- Math.pi;
stdIn:38.1-38.8 Error: unbound variable or constructor: pi in path Math.pi