ExternalSymbol

The ExternalSymbol is a symbol node that represents a symbol declared in another module.

Declaration

Syntax

ExternalSymbol(symbol_table parent_symtab, identifier name,
        symbol external, identifier module_name, identifier* scope_names,
        identifier original_name, access access)

Arguments

Argument Name	Denotes
parent_symtab	the parent symbol table that contains the external symbol
name	the name of the external symbol in the current symbol table
external	pointer to the actual symbol definition
module_name	the name of the module the symbol is in
scope_names	a list of names if the symbol is in a nested symbol table. For example if it is a local variable in a function f that is nested in function g, then scope_names=[g, f]
original_name	the name of the symbol in the external symbol table
access	access type Public/Private

Return values

None.

Description

ExternalSymbol represents symbols that cannot be looked up in the current scoped symbol table. As an example, if a variable is defined in a module, but used in a nested subroutine, that is not an external symbol because it can be resolved in the current symbol table (nested subroutine) by following the parents. However if a symbol is used from a different module, then it is an external symbol, because usual symbol resolution by going to the parents will not find the definition. The ExternalSymbol is the only way to reference a symbol that cannot be accessed in the scoped symbol table by visiting the parents. There is a special handling for it in the serialization and deserialization: the external member is not serialized (since it is a pointer) and in deserialization the pointer is reconstructed from the original_name and scope_names.

The scope_names contains the names of the external symbol table starting from the top how to get to the symbol. This approach allows to reference any nested symbol (such as a local variable in a function in a module). However, we might later change the design to only allow referencing top level module entities.

One can think of the ExternalSymbol as the « import » statement in Python, or the « use » statement in Fortran.

Types

Examples

module module_num
    integer :: my_num = 5
end module

program main
  use module_num
  print *, my_num
end program

ASR:

(TranslationUnit
    (SymbolTable
        1
        {
            main:
                (Program
                    (SymbolTable
                        3
                        {
                            my_num:
                                (ExternalSymbol
                                    3
                                    my_num
                                    2 my_num
                                    module_num
                                    []
                                    my_num
                                    Public
                                )
                        })
                    main
                    [module_num]
                    [(Print
                        ()
                        [(Var 3 my_num)]
                        ()
                        ()
                    )]
                ),
            module_num:
                (Module
                    (SymbolTable
                        2
                        {
                            my_num:
                                (Variable
                                    2
                                    my_num
                                    []
                                    Local
                                    (IntegerConstant 5 (Integer 4 []))
                                    ()
                                    Save
                                    (Integer 4 [])
                                    Source
                                    Public
                                    Required
                                    .false.
                                )
                        })
                    module_num
                    []
                    .false.
                    .false.
                )
        })
    []
)

See Also

symbol.