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18. Scopes & symbolic names

18. Scopes & symbolic names

The last formal chapter answers: where is a name known, and what does it mean? FORTRAN 77's scoping is simple — there are no nested procedures and no blocks — so a name is either global (known across the whole program) or local (known in one program unit) (§18).

Global vs local

Global entities are known throughout the executable program, and their names must be unique across it (§18.1.1):

  • common-block names,
  • external function and subroutine names,
  • the main program name and block-data names.

Local entities belong to a single program unit; the same name may mean different things in different units (§18.1.2):

  • variables, arrays, named constants (PARAMETER),
  • statement functions, intrinsic functions, dummy procedures.

So X in one subroutine and X in another are unrelated variables; they share data only through arguments or COMMON. A few names have an even narrower scope: a statement function's dummy arguments, and a DATA implied-DO variable, are local to that one statement.

Classes, and "no reserved words"

Because FORTRAN has no reserved words, the compiler decides what a name is — its class — from context (§18.2). The same spelling can be a keyword in one place and your variable in another:

      INTEGER INDEX            ! INDEX is now this unit's variable...
      INDEX = 5
      K = INDEX(STR, 'X')      ! ...and INDEX the intrinsic elsewhere is a different unit's affair

The resolution rules, informally:

  • A name followed by ( in an expression is a function reference or an array element — decided by whether the name was declared an array.
  • An intrinsic-function name can be shadowed by a dummy argument, a declared variable, or an EXTERNAL/INTRINSIC declaration.
  • A common-block name (it lives in /…/) may also be used as a local variable or array name — they don't collide.
  • A function's own name doubles as the result variable inside the function (Chapter 15).

The one firm rule: a name must not belong to two local classes in the same unit — you cannot use FOO as both a variable and a statement function, or assign to a PARAMETER constant.

forterp notes. forterp resolves name classes by context exactly this way (no reserved words), so an intrinsic name used as a variable — the common case — works, and is exercised throughout the conformance corpus. The general "one name, one local class" prohibition is not comprehensively diagnosed (a conforming program never violates it), but its one common, harmful instance — assigning to a PARAMETER — is a hard error rather than a silently-dropped assignment (Chapter 8). Names longer than six characters are accepted (first six significant) rather than rejected; see Appendix D.