Check-in [78a116deef]

{"tip": {
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<th>Type</th>
<th>Tcl Version</th>
<th>Status</th>
<th>Title</th>
<th>Impl.</th>
</tr></thead><tbody>

<tr class='state-draft type-project version-87'>
<td valign='top'><a href='./tip/551.md'>551</a></td>
<td valign='top'>Project</td>
<td valign='top'>8.7</td>
<td valign='top'>Draft</td>
<td valign='top' ><a href='./tip/551.md'>permit underscore in numerical literals in source code</a></td>
<td></td>
</tr>
<tr class='state-draft type-project version-90'>
<td valign='top'><a href='./tip/550.md'>550</a></td>
<td valign='top'>Project</td>
<td valign='top'>9.0</td>
<td valign='top'>Draft</td>
<td valign='top' ><a href='./tip/550.md'>Garbage Collection for TclOO</a></td>
<td></td>

# TIP 551: permit underscore in numerical literals in source code
    Author:        [email protected]
    State:         Draft
    Type:          Project
    Vote:          Pending
    Created:       16-Sep-2019
    Post-History:
    Tcl-Version:   8.7
    Keywords:      numbers, readability
    Sponsor:       Brian Griffin <[email protected]>
    
# Abstract

This TIP proposes that all numbers in scripts shall allow underscores
for readability of code.

# Rationale and Discussion

In most modern programming languages, it is possible to group the
digits in the program's source code to make it easier to read; Ada, C#
(from version 7.0), D, Haskell (from GHC version 8.6.1), Java, OCaml,
Perl, Python (from version 3.6), Ruby, Rust, and Swift all use the
underscore (`_`) character for this purpose. All these languages allow
nine hundred million to be entered as `900_000_000`.

This TIP proposes to change TCL to include this ability.

# Specification

To allow underscore in numerical constants (decimal, octal, 0x... 0b...
0d... and 0...) the underscore character would be simply an aid to
visibility. This character would serve as a comment in the sense it
would be allowed in the program source code but have no semantic
affect. Any number of underscores would be allowed and their positions
would be unrestricted except where a number would no longer be a
number.

For example,

    `expr 100_000_000`
    `expr 0xffff_ffff`
    `expr 0b1111_1111_1111_1110`

A number is identified in the routine TclParseNumber and scans the
text until the end of the number. It accepts several formats. The key
is that a number is a word that begins with a decimal digit and can
have several forms for decimal, octal, hexidecimal, and binary. The
convention is now to use a 2 character prefix, 0x, 0b, 0o, and the
newest one, 0d. This proposal would allow an underscore following the
first digit or the letter that designates the number base, to anywhere
up to and including the end of the number. There can be multiple
underscores in a row.

Thus,

    `0x_ffff_ffff_`

would be legal and similarly with the other 3 bases supported.

The one restriction would be a number with a leading `_` as that would
change the meaning from a number to a bareword, e.g.

    `expr _123`

would not be allowed, since that would not be a number.

The implementation would be to simply bypass the `_` in all cases during
the processing of the number in the routine TclParseNumber.

The implementation was tested with 4 lines of code at the beginning of
the function. It is at the main while loop in TclParseNumber and
includes this,


    while (1) {
      char c = len ? *p : '\0';
    
      // ------------- add this to allow _ in a number and just bypass it
      if ( c == '_' ) {
        p++;
        continue;
      }
      // -------------- end of code to allow underscore
      ...
    }

# Copyright

This document has been placed in the public domain.