Mstrings - Pretty Multliline Strings for Common Lisp

Mstrings defines a reader macro for strings with a small handful of features to provide visually appealing multiline blocks. An M-string:

• Trims leading whitespace from lines
• Respects escaped whitespace characters and keeps them untrimmed
• Concatenates lines together when joined by an escaped Newline
• Alternative mode to fold multiple non-empty lines into one joined by spaces

When writing strings that take multiple lines, for example an extensive docstring, it feels natural to align them up. However, the Lisp reader will keep all leading whitespace in the string:

* (defun grab-config (use-default)
    (if use-default
        "foo=1
         bar=2
         qux=3"
        (read-file-into-string +config-filepath+)))

* (grab-config t)
"foo=1
         bar=2
         qux=3"

The workaround is to shove all subsequent lines to the beginning of the file, which could appear quite unsightly:

* (defun grab-config (use-default)
    (if use-default
        "foo=1
bar=2
qux=3"
        (read-file-into-string +config-filepath+))

* (grab-config t)
"foo=1
bar=2
qux=3"

M-strings allow a plain method to remove the whitespace while keeping content aligned in the source code:

* (defun grab-config (use-default)
    (if use-default
        #M"foo=1
           bar=2
           qux=3"
        (read-file-into-string +config-filepath+)))

* (grab-config t)
"foo=1
bar=2
qux=3"

Contributions

Any comments, questions, issues, or patches are greatly appreciated! I do my main development on Sourcehut, with a mailing list and issue tracker.

Install and Enable It

M-strings' sole dependency is named-readtables, available on Quicklisp.

Install M-strings locally, until it is added to Quicklisp:

$ cd ~/common-lisp/ # Or wherever you store your systems
$ git clone https://git.sr.ht/~shunter/mstrings

This library uses named readtables to expose the reader:

* (use-package :named-readtables)
* (in-readtable mstrings:mstring-syntax)

* (princ #M"Hello
            World!")
Hello
World!

Features

M-strings trim leading whitespace from lines:

* (princ #M"Hello
            World!")
Hello
World!

M-strings respect escaped characters and keeps them untrimmed:

* (princ #M"keys:
           \  foo: \"apple\"
           \  bar: \"banana\"")
keys:
  foo: "apple"
  bar: "banana"

M-strings concatenates lines together when they're joined by an escaped Newline - useful for very long single-line values:

* (princ #M"NB2HI4DTHIXS653XO4XHS33VOR2WEZJOMNXW\
            2L3XMF2GG2B7OY6WIULXGR3TSV3HLBRVC===")
NB2HI4DTHIXS653XO4XHS33VOR2WEZJOMNXW2L3XMF2GG2B7OY6WIULXGR3TSV3HLBRVC===

By default, M-strings are read in "literal-block mode", where newlines are read as literal newlines. Prefixing a string with a > sets the reader to "folding-block mode", where multiple non-empty lines are folded into one, joined by spaces:

* (princ #M>"The quick brown fox
             jumps over
             the lazy dog.

             Sphinx of black quartz,
             judge my vow!")
The quick brown fox jumps over the lazy dog.
Sphinx of black quartz, judge my vow!

* (princ #M>"Escaped newlines
             \
             still prevent
             \
             line breaks")
Escaped Newlines still prevent line breaks

Shorthand notation: #"..." and #>"..."

The reader macro function understands #"..." and #>"..." as shorthands for #M"..." and #M>"...", respectively. They're defined separately to prevent macro collisions from other systems:

* (in-readtable mstrings:shorthand-mstring-syntax)
* ;; Or, for both shorthand and long-hand:
* (in-readtable mstrings:full-mstring-syntax)

*    #"Literal-block
       Mode"
"Literal-block
Mode"
*    #>"Folding-block
        Mode"
"Folding-block Mode"

Be aware, #" is also used by the string-escape library, and #> is used by Clozure CL 1.2 and later (and therefore elided from the shorthand-mstrings syntax).

API

[Function] mstring-reader stream subchar arg

Reader macro accepts multiline strings. It ignores and warns on any provided arg, and provides a few quality-of-life features depending on the value of subchar:

• If subchar is #", it is always literal-block mode (#M"...") and reads the rest of the string.
• If subchar is #>, it is always folding-block mode (#M>"...").
• If subchar is anything else, it follows the default behavior: it assumes literal-block mode unless there is a greater-than-sign preceding the string, in which case it switches to folding-block mode.

[Readtable] mstring-syntax

The standard readtable with the longhand mstring syntax:

• #M"..." - Literal-block mstring
• #M>"..." - Folding-block mstring

[Readtable] shorthand-mstring-syntax

The standard readtable with both #" and #> dispatch character pairs bound to mstring-reader:

• #"..." - Literal-block mstring
• #>"..." - Folding-block mstring

[Readtable] full-mstring-syntax

The standard readtable with additions from both mstring-syntax and shorthand-mstring-syntax:

• #M"...", #"..." - Literal-block mstring
• #M>"...", #>"..." - Folding-block mstring