Triple-quoted multiline strings with trim margin: '''...'''

[tmteam]

Summary

Introduce triple-quoted strings '''...''' (and """...""") for multiline text with automatic indentation trimming (trim margin).

Dialect setting

enum MultilineString
{
    Deny,     // '''...''' not recognized
    Allow,    // '''...''' with trim margin + $-prefix escape levels
}

Default: Deny (current behavior). Allow enables the full feature set including $'''...''' escape levels (#99).

Syntax

sql = '''
    SELECT *
    FROM users
    WHERE age > 18
    '''
# → "SELECT *\nFROM users\nWHERE age > 18"

Trim margin algorithm (Swift/Java/C# style)

1. First newline after opening ''' is stripped
2. Last newline before closing ''' is stripped
3. Find the minimum common whitespace prefix across all non-empty content lines and the closing ''' line
4. Strip that prefix from every line
5. Tabs and spaces are compared character-by-character (mixed = error or no trim)

Rules

• Opening ''' must be followed by a newline (nothing else on that line)
• Closing ''' determines the baseline indentation
• A content line with less indentation than closing ''' → compile error
• Both ''' and """ are supported (equivalent)
• Interpolation uses {expr} — same as single-quoted strings
• Escape sequences (\n, \t, \{, etc.) work as usual
• To include literal ''' inside: use \''' or switch to """

Escape level prefix ($-prefix)

When MultilineString.Allow is set, $-prefix is also available (for both '...' and '''...'''):

Syntax	Interpolation	{ literal	$ literal
'''...'''	{expr}	\{	literal
$'''...'''	${expr}	literal	\$
$$'''...'''	$${expr}	literal	literal

# JSON — braces literal, interpolation via ${expr}
json = $'''
    {"name": "${name}", "age": ${age}}
    '''

# Template with $ signs — interpolation via $${expr}
report = $$'''
    Price: $100
    Customer: $${customer}
    '''

Edge cases

• Empty triple-quote: '''\n''' → ""
• Single line: '''hello''' → error? Or treat as regular string? (to decide)
• Tabs vs spaces mixed indentation → compile error

Related

• Depends on: Dialect setting: disallow newlines in single-quoted strings #97 (dialect setting for single-line strings)
• Subsumes: Escape level prefix for string interpolation: $'...' $$'...' #99 (escape levels — included in this feature when Allow)

[tmteam]

Design discussion: lessons from other languages

Dialect setting not needed

''' is currently invalid syntax in NFun ('' + ' = parse error). Enabling triple-quoted strings doesn't break any existing code — it's purely additive. Unlike AllowNewlineInStrings or AllowUserFunctions, there's no reason to deny this feature. No dialect setting needed — just enable by default.

Newline after opening ''' — required

Without this requirement, trim margin rules become ambiguous:

# Case 1: what's the baseline? First line has no indent
"""hello
   world
   """

# Case 2: closing """ not on its own line — what's the baseline?
"""
   hello
   world"""

When both opening and closing are on their own lines — rules are unambiguous:

"""
   hello
   world
   """
# baseline = indent of closing """ = 3 spaces
# result: "hello\nworld"

Only ''' and """ — no 4+ quotes

C# allows """", """"" etc for embedding quotes in content. Not needed — use """ when content has ''' and vice versa. Two kinds of quotes cover all practical cases.

Six simple rules (lessons from C# complexity)

1. Only ''' and """ — no 4+ quotes
2. Newline required after opening '''
3. Closing ''' must be on its own line
4. Trim rule: indent of closing ''' = baseline, strip from all lines
5. Content line with less indent than baseline → compile error
6. Mixed tabs and spaces → compile error

No special cases, no exceptions.

Complaints from other languages

Problem	Python	Kotlin	C#
Trim margin forgotten (library call)	Yes	Yes	No (built-in)
Indentation rules confusing	—	—	Yes
Auto-formatter breaks strings	—	—	Yes
Mixed tabs/spaces issues	—	—	Yes

Conclusion for NFun: trim margin must be built into syntax (not a method call), rules must be minimal, mixed whitespace = error.