Node:implementation constructor destructor operator uses import initialization, Previous:forward near far, Up:Parsing keywords

#### `implementation`, `constructor`, `destructor`, `operator`, `uses`, `import` and `initialization` as weak keywords

In ISO 7185 Pascal, each section of the source code is uniquely introduced by a keyword (`program`, `const`, `type`, `var`, `label`, `procedure`, `function`, `begin`). However, the ending of some of these sections (in particular `const`, `type` and `var`) is not intrinsically defined, but only by the context (the next of these "critical" keywords). E.g., `var Foo: Integer;` can be a complete variable declaration part (if one of those keywords follows), or only a part of one, as in `var Foo: Integer; Bar: Integer;`. (For the other keywords, the ending is intrinsically defined - the `program` heading and `label` declarations end with the next `;`. For `procedure` and `function` it's a little more complicated, due to `forward` declarations, but still well-defined, and `begin` ends with the matching `end`). The same applies to sections within one routine, except that `program` cannot occur there.

Extended Pascal adds `to` (in `to begin do` and `to end do`) and `end` (in interface modules and implementation modules without initializer and finalizer) to those "critical" keywords.

But it also adds two keywords which are not defined in classic Pascal, namely `export` and `import`. But they can only occur at the beginning of the source or of a module implementation so they have fewer chances to conflict with those other keywords. The same applies to UCSD/Borland Pascal's `uses` for units. (`uses` terminates at the first `;`, `export` and `import` do not necessarily, like `var` etc.)

The problem gets bigger with UCSD/Borland Pascal's `implementation` in units. It can occur after the interface part, so it might follow, e.g., a variable declaration part. And it is not an ISO 7185 Pascal keyword.

If we want to treat `implementation` as a weak keyword, it must not conflict with new identifiers anywhere in the grammar.

However, variable declaration parts are not self-contained in the sense described above, so after a variable declaration part it is not immediately clear if the part is finished or will continue. So this is a place where a new identifier is acceptable. E.g.:

    interface

    var
      Bar: Integer;
      Implementation: Integer;

vs.

    interface

    var
      Bar: Integer;

    implementation

The same applies to `implementation` after `const`, `type`, `export` and `import` parts.

The same problem also occurs with the Borland Pascal and Object Pascal keywords `constructor` and `destructor`, the Borland Delphi keyword `initialization`, and the PXSC keyword `operator` since the respective declarations can follow variable declaration blocks etc. It also happens with `import` (but it is only possible after an `export` part) and with `uses` if we allow it after other declarations (GPC extension).

Again, we play some lexer tricks. We observe that the new identifier in `export`, `var`, `const` and `type` is always followed by either `,`, `:` or `=` while none of the keywords `implementation`, `constructor`, `destructor`, `operator`, `import` and `uses` is ever followed by one of these symbols ... with two exceptions: `operator =` is valid, overloading the `=` operator. Consider:

    type
      Foo = record end;
      Operator = (a, b);  { enum type }

vs.

    type
      Foo = record end;

    operator = (a, b: Foo) c: Foo;

To decide whether `operator` is a keyword, we would have to look ahead six tokens! Anyway, that seems to be a new record (where "record" in this sentence can be read either as a Pascal keyword or in at least one of the usual English meanings ;-).

The other exception is that `initialization` can, in principle, be followed by `(`, as in:

    implementation

    type
      Foo = Integer;
      Initialization (Obj: Integer)

vs.

    implementation

    type
      Foo = Integer;

    Initialization
      (Obj as SubObj).Method;

This would require 3 tokens look-ahead. However, a `(` at the beginning of a statement is quite uncommon, so we just disallow that, so the use of `Initialization` as an identifier is not restricted.

Doing so much look-ahead would be a huge effort and cause some complications as noted above. This seems inappropriate for such an academic example. So, until someone comes up with a clever trick to cope with this case, we give up here and let `operator` before `=` be a keyword, so overloading `=` is possible. This means that `operator` cannot be used as an `export` interface, a type or an (untyped) constant, unless the keyword is disabled explicitly or by dialect options. (Enabling and disabling the keyword by the parser would also have been no option here, since the parser would need the 6-token look-ahead just as well, which it cannot do.)

You may have noticed that we "forgot" `import` (in the list of possibly unfinished sections; not in the list of critical following keywords where it was alright; it actually plays both roles in this discussion).

This is because the identifier at the beginning of an import specification can be followed by `qualified`, `only`, `in`, `(` or `;` - the former two of which are non-standard keywords as well and would therefore conflict with a new identifier after, e.g., `uses` and `operator`.

This means that there's no simple general solution. So let's consider the problematic keywords after an `import` part in detail:

- `import`. Can't happen since EP only allows only `import` part (possibly containing multiple import specifications). So this one doesn't cause a S/R conflict, unlike the following ones.
- `uses`. Combining module-style `import` with unit-style `uses` is a direct mix of different standards. According to the discussion above, it would lead to the following ambiguity:

      import Foo; Uses only (a);  { import only a from Uses }

  vs.

      import Foo;

      uses Only (a);  { import a from Only }

  Though `uses` with an import-list is another "cross-standard" extension, disallowing it would only reduce the issue from an ambiguity to a two-token look-ahead conflict and not really help much - whereas it would devalue the usefulness of `uses` which otherwise can always serve as a substitute for `import`, e.g. to avoid all the conflicts discussed here (because `uses` is terminated by the first `;`).

- `operator`.

      import Foo; Operator only (a, b);

  (i.e., import only `a` and `b` from an interface called `Operator`), vs.

      import Foo;

      operator Only (a, b: Integer) c: Integer;

  As in the case of `operator =`, we would need 6 tokens of look-ahead. We have to give up.

- `implementation`. This does not happen for module implementations since their syntax is different (`module Foo implementation;`), but for unit implementations. Combining these with module-style `import` is therefore "cross-standard" already. In addition, it would imply an empty interface part (apart from the imports) which is rather pointless in units (whereas it might be useful in modules, containing only re-exports, but as noted, module implementations are unproblematic here).
- `constructor` and `destructor`. In an interface, these actually do not make sense immediately after `import` since their purpose is to implement constructors and destructors of object types that must have been declared before (not imported). But it could happen in an implementation.

We forbid all of these keywords immediately after an `import` part. This is achieved using parser precedence rules.