CODEWITHSUNDEEP
smlml
Tim
Tim

Reputation: 99526

Is a structure declaration a declaration in the core language or the module language?

In SML's grammar

Core Language

dec ::=   val ⟨var⟩(,) valbind value
          fun ⟨var⟩(,) funbind function
          type typbind type
          datatype datbind ⟨withtype typbind⟩ data type
          datatype id = datatype longid data type replication
          abstype datbind ⟨withtype typbind⟩ with dec end abstract type
          exception exnbind exception
          structure strbind structure (not allowed inside expressions)
          empty

Module Language

Programs

prog ::=   dec core declaration
           functor fctbind functor declaration
           signature sigbind signature declaration
           empty
           prog1 ⟨;⟩ prog2 sequence

fctbind ::= id1 ( id2 : sig ) ⟨:⟨>⟩ sig⟩ = str ⟨and fctbind⟩    plain
id ( spec ) ⟨:⟨>⟩ sig⟩ = str ⟨and fctbind⟩ opened
sigbind    ::= id = sig ⟨and sigbind⟩ signature

Are structure, signature and functor all concepts in the module language, not in the core language?

I was wondering why does a structure declaration structure strbind enter a program as a declaration dec through the core language, instead of the module language?

In other words, why is a structure declaration structure strbind classified as a declaration dec in the core language, instead of the module language, unlike signature declarations signature sigbind and functor declarations functor fctbind?

Upvotes: 0

Views: 57

Answers (1)

Andreas Rossberg
Andreas Rossberg

Reputation: 36118

Structure declarations are part of the module system. But they are allowed inside structure expressions, unlike functor and signature declarations. None of them is allowed inside the core level.

Technically, there are three levels of declarations in the actual language specification: core dec, structure dec, toplevel dec. The grammar you're looking at simplifies that a little, to avoid too much duplication.

Upvotes: 1

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