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6.10 Module types (module specifications)

Module types are the module-level equivalent of type expressions: they specify the general shape and type properties of modules.

module-type	::=	modtype-path
	|	sig { specification  [;;] } end
	|	functor ( module-name :  module-type ) ->  module-type
	|	module-type with  mod-constraint  { and mod-constraint }
	|	( module-type )
specification	::=	val value-name :  typexpr
	|	external value-name :  typexpr =  external-declaration
	|	type-definition
	|	exception constr-decl