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21.2.11 Gotchas (Transforming)

This section is not about transforming “gotchas” into something else. It is about the weirder aspects of transforming Fortran, however that's defined, into a more modern, canonical form.

21.2.11.1 Multi-character Lexemes

Each lexeme carries with it a pointer to where it appears in the source.

To provide the ability for diagnostics to point to column numbers, in addition to line numbers and names, lexemes that represent more than one (significant) character in the source code need, generally, to provide pointers to where each character appears in the source.

This provides the ability to properly identify the precise location of the problem in code like

     SUBROUTINE X
     END
     BLOCK DATA X
     END

which, in fixed-form source, would result in single lexemes consisting of the strings SUBROUTINEX and BLOCKDATAX. (The problem is that X is defined twice, so a pointer to the X in the second definition, as well as a follow-up pointer to the corresponding pointer in the first, would be preferable to pointing to the beginnings of the statements.)

This need also arises when parsing (and diagnosing) FORMAT statements.

Further, it arises when diagnosing FMT= specifiers that contain constants (or partial constants, or even propagated constants!) in I/O statements, as in:

     PRINT '(I2, 3HAB)', J

(A pointer to the beginning of the prematurely-terminated Hollerith constant, and/or to the close parenthese, is preferable to a pointer to the open-parenthese or the apostrophe that precedes it.)

Multi-character lexemes, which would seem to naturally include at least digit strings, alphanumeric strings, CHARACTER constants, and Hollerith constants, therefore need to provide location information on each character. (Maybe Hollerith constants don't, but it's unnecessary to except them.)

The question then arises, what about other multi-character lexemes, such as ** and //, and Fortran 90's (/, /), ::, and so on?

Turns out there's a need to identify the location of the second character of these two-character lexemes. For example, in I(/J) = K, the slash needs to be diagnosed as the problem, not the open parenthese. Similarly, it is preferable to diagnose the second slash in I = J // K rather than the first, given the implicit typing rules, which would result in the compiler disallowing the attempted concatenation of two integers. (Though, since that's more of a semantic issue, it's not that much preferable.)

Even sequences that could be parsed as digit strings could use location info, for example, to diagnose the 9 in the octal constant O'129'. (This probably will be parsed as a character string, to be consistent with the parsing of Z'129A'.)

To avoid the hassle of recording the location of the second character, while also preserving the general rule that each significant character is distinctly pointed to by the lexeme that contains it, it's best to simply not have any fixed-size lexemes larger than one character.

This new design is expected to make checking for two * lexemes in a row much easier than the old design, so this is not much of a sacrifice. It probably makes the lexer much easier to implement than it makes the parser harder.

21.2.11.2 Space-padding Lexemes

Certain lexemes need to be padded with virtual spaces when the end of the line (or file) is encountered.

This is necessary in fixed form, to handle lines that don't extend to column 72, assuming that's the line length in effect.

21.2.11.3 Bizarre Free-form Hollerith Constants

Last I checked, the Fortran 90 standard actually required the compiler to silently accept something like

     FORMAT ( 1 2   Htwelve chars )

as a valid FORMAT statement specifying a twelve-character Hollerith constant.

The implication here is that, since the new lexer is a zero-feedback one, it won't know that the special case of a FORMAT statement being parsed requires apparently distinct lexemes 1 and 2 to be treated as a single lexeme.

(This is a horrible misfeature of the Fortran 90 language. It's one of many such misfeatures that almost make me want to not support them, and forge ahead with designing a new “GNU Fortran” language that has the features, but not the misfeatures, of Fortran 90, and provide utility programs to do the conversion automatically.)

So, the lexer must gather distinct chunks of decimal strings into a single lexeme in contexts where a single decimal lexeme might start a Hollerith constant.

(Which probably means it might as well do that all the time for all multi-character lexemes, even in free-form mode, leaving it to subsequent phases to pull them apart as they see fit.)

Compare the treatment of this to how

     CHARACTER * 4 5 HEY

and

     CHARACTER * 12 HEY

must be treated—the former must be diagnosed, due to the separation between lexemes, the latter must be accepted as a proper declaration.

21.2.11.4 Hollerith Constants

Recognizing a Hollerith constant—specifically, that an H or h after a digit string begins such a constant—requires some knowledge of context.

Hollerith constants (such as 2HAB) can appear after:

Hollerith constants don't appear after:

21.2.11.5 Confusing Function Keyword

While

     REAL FUNCTION FOO ()

must be a FUNCTION statement and

     REAL FUNCTION FOO (5)

must be a type-definition statement,

     REAL FUNCTION FOO (names)

where names is a comma-separated list of names, can be one or the other.

The only way to disambiguate that statement (short of mandating free-form source or a short maximum length for name for external procedures) is based on the context of the statement.

In particular, the statement is known to be within an already-started program unit (but not at the outer level of the CONTAINS block), it is a type-declaration statement.

Otherwise, the statement is a FUNCTION statement, in that it begins a function program unit (external, or, within CONTAINS, nested).

21.2.11.6 Weird READ

The statement

     READ (N)

is equivalent to either

     READ (UNIT=(N))

or

     READ (FMT=(N))

depending on which would be valid in context.

Specifically, if N is type INTEGER, READ (FMT=(N)) would not be valid, because parentheses may not be used around N, whereas they may around it in READ (UNIT=(N)).

Further, if N is type CHARACTER, the opposite is true—READ (UNIT=(N)) is not valid, but READ (FMT=(N)) is.

Strictly speaking, if anything follows

     READ (N)

in the statement, whether the first lexeme after the close parenthese is a comma could be used to disambiguate the two cases, without looking at the type of N, because the comma is required for the READ (FMT=(N)) interpretation and disallowed for the READ (UNIT=(N)) interpretation.

However, in practice, many Fortran compilers allow the comma for the READ (UNIT=(N)) interpretation anyway (in that they generally allow a leading comma before an I/O list in an I/O statement), and much code takes advantage of this allowance.

(This is quite a reasonable allowance, since the juxtaposition of a comma-separated list immediately after an I/O control-specification list, which is also comma-separated, without an intervening comma, looks sufficiently “wrong” to programmers that they can't resist the itch to insert the comma. READ (I, J), K, L simply looks cleaner than READ (I, J) K, L.)

So, type-based disambiguation is needed unless strict adherence to the standard is always assumed, and we're not going to assume that.