Grammar for the Moscow ML module language

The Moscow ML module language is much simplified compared to the full Standard ML Modules language.

The equivalent of a Standard ML structure is a unit. A unit U consists of a unit interface in file U.sig (corresponding to an SML signature) and a unit body in file U.sml (corresponding to an SML structure). The unit signature can be left out.

Unit Body (in file unitid.sml)

Unit Signature (in file unitid.sig)

Syntactic restrictions

• In Moscow ML, the unitid, if specified in a unit signature or unit body file, must agree with the filename. The name of the constraining signature, if any, must equal that of the structure.
• No specification valdesc, typdesc, typbind, datdesc, or exdesc may describe the same identifier twice; this applies also to value constructors within a datdesc.
• In the left side tyvarseq tycon in any typdesc, typbind, or datdesc, tyvarseq must not contain the same tyvar twice. Any tyvar occurring within the right side of a typbind or datdesc must occur in tyvarseq.
• No type, value, or exception may be specified twice at top-level in a signature.
• No datdesc or exdesc may specify true, false, it, nil, ::, or ref.