Nestable pragmats in Algol 68
*****************************

                        GNU68-2026-002 (draft)

Copyright (C) 2026 Jose E. Marchesi.

     You can redistribute and/or modify this document under the terms of
     the GNU General Public License as published by the Free Software
     Foundation, either version 3 of the License, or (at your option)
     any later version.

The following specification has been released under the auspices of the
GNU Algol 68 Working Group, and has been scrutinized to ensure that

  a. it is strictly upwards-compatible with Algol 68,
  b. it is consistent with the philosophy and orthogonal framework of
     the language, and
  c. it fills a clearly discernible gap in the expressive power of that
     language.

The source of this document can be found at
<https://git.sr.ht/~jemarch/gnu68>.

The informal description of this proposal introduces the proposed new
language features, providing a rationale and usage examples.

The formal definition of this proposal uses the existing formalism and
conventions of the Revised Report, and it is expressed as modifications
to the Report.

Finally, the implementation notes of this proposal describes a way in
which the features added by this specification can be implemented.  No
implementer should feel committed to do things as described there; the
same language facilities may well be implementable in other ways, more
suitable to specific implementations.

1 Informal Description
**********************

Pragmats in Algol 68
====================

The Algol 68 strict language defines two styles for pragmats, each style
using a different delimiter symbol:

'bold pragmat symbol'                'PRAGMAT'
'style i pragmat symbol'             'PR'

In both cases the same symbol marks the beginning and end of the
pragmat: a bold pragmat starts with 'PRAGMAT' and it must be ended by
another 'PRAGMAT'; a 'style i' pragmat starts with 'PR' and ends with a
'PR'.  These delimiters are not ideal, for several reasons.

First, using the same symbol for starting and ending a pragmat makes it
necessary to inspect the whole source in order to determine, given a
delimiter, whether it begins or finishes the construct.  For programmers
this can be confusing and error prone, for programs such as text editors
wanting to highlight a pragmat, or to skip over it, this can be very
inefficient.

Pragmats using that kind of delimiters cannot be nested.  This may not
seem like a significant limitation at first.  Other constructs, such as
comments, are often nested, but not annotations for the compiler.
However, the ability of doing so may be useful in order to combine
several pragmats without having to resort to other language constructs.

Nestable pragmats in Algol 68
=============================

The shortcomings described in the previous section motivate the addition
of nestable pragmat delimiters to Algol 68.

Representations of pragmats are defined for the "bold" and "i" styles,
both of which use symmetric delimiters:

'bold pragmat begin symbol'          'ORDER'
'bold pragmat end symbol'            'REDRO'
'style i pragmat begin symbol'       '[['
'style i pragmat end symbol'         ']]'

In Algol 68 the contents of several contiguous pragmats (ignoring
possibly intermixed comments) are combined via simple concatenation into
a string that is then interpreted as a single pragmat.

For example, the following pragmats:

     [[un]] [[used]]

are equivalent to:

     [[unused]]

When nested pragmats are found, they are expanded into the contents of
the containing pragmat, including both delimiters and contents.  This
makes it possible for pragmats to operate on other pragmats.  For
example, the following pragmats may be a no-op:

     [[unused]] [[ignore [[unused]]]]

2 Formal Description
********************

New symbols have been invented for opening and closing pragmats:

     9.4.1.h
       bold pragmat begin symbol                   ORDER
       bold pragmat end symbol                     REDRO
       style i pragmat begin symbol                [[
       style i pragmat end symbol                  ]]

Syntax of pragments is expanded:

     9.2.1

     b) PRAGMENT{a} : ... ; STYLE nestable pragmat{d}.
     d) STYLE nestable pragmat{b} :
          STYLE pragmat begin symbol{94h,-},
            STYLE PRAGMENT item{c} sequence option,
            STYLE pragmat end symbol{94h,-}.

3 Implementation Notes
**********************

The new pragmats are introduced as an addition to the existing pragmats,
requiring rather straightforward changes to the lexical analyzer.

It is recommended for implementations offering stropping regimes based
on reserved words to only recognize pragmats with symmetric delimiters,
and of these only the brief style.

Since pragmats can be concatenated, it is advisable to compilers to use
a pragmat language in which such concatenation will lead to a valid
sequence of orders.