5.9.5 Expressions

An expression is any value that can be assigned to an attribute or a variable. It is either a literal value, or a construct requiring runtime computation by the project manager. In a project file, the computed value of an expression is either a string or a list of strings.

A string value is one of:

• A literal string, for instance "comm/my_proj.gpr"
• The name of a variable that evaluates to a string (see Variables)
• The name of an attribute that evaluates to a string (see Attributes)
• An external reference (see External Values)
• A concatenation of the above, as in "prefix_" & Var.

A list of strings is one of the following:

• A parenthesized comma-separated list of zero or more string expressions, for instance (File_Name, "gnat.adc", File_Name & ".orig") or ().
• The name of a variable that evaluates to a list of strings
• The name of an attribute that evaluates to a list of strings
• A concatenation of a list of strings and a string (as defined above), for instance ("A", "B") & "C"
• A concatenation of two lists of strings

The following is the grammar for expressions

    string_literal ::= "{string_element}"  --  Same as Ada
    string_expression ::= string_literal
        | *variable_*name
        | external_value
        | attribute_reference
        | ( string_expression { & string_expression } )
    string_list  ::= ( string_expression { , string_expression } )
       | *string_variable*_name
       | *string_*attribute_reference
    term ::= string_expression | string_list
    expression ::= term { & term }     --  Concatenation

Concatenation involves strings and list of strings. As soon as a list of strings is involved, the result of the concatenation is a list of strings. The following Ada declarations show the existing operators:

    function "&" (X : String;      Y : String)      return String;
    function "&" (X : String_List; Y : String)      return String_List;
    function "&" (X : String_List; Y : String_List) return String_List;

Here are some specific examples:

    List := () & File_Name; --  One string in this list
    List2 := List & (File_Name & ".orig"); -- Two strings
    Big_List := List & Lists2;  --  Three strings
    Illegal := "gnat.adc" & List2;  --  Illegal, must start with list