Change raw string literal syntax: [#]\*" represents single-line string and [#]\*''' represents block string
Table of contents
Problem
Under current design of string literals, users may make assumptions that a starting [#]*""" represents a block string and misunderstand the syntax.
Background
The design of string literals specifies a block string literal to start with [#]*""". Users may take for granted the other way around, where any string literal starting with [#]*""" is a block string literal. This does not hold true, however. Two counter-cases are """abc""" represents three tokens "", "abc" and "", and #"""# which is equivalent to "\"". Neither is a block string literal and may be visually confusing.
Proposal
Interpret [#]*" as the start of single-line string literals, and [#]*''' as the start of block string literals. Disallow adjacent string literals like """abc""".
Details
Users can easily distinguish single-line string literals from block string literals with the proposed change. Confusion on """abc""" will be eliminated because adjacent string literals are invalid in the proposal. On the other hand, #"""# will be clear to the user of representing "\"", as """ does not represent a block string literal any more. More details can be found here.
Rationale
This principle helps make Carbon code easy to read, understand, and write, because it avoids confusion on the type of certain string literals.
Alternatives considered
The current implementation
In addition to the confusion described above, the current implementation complicates the lexing. When the lexer sees [#]+""", it temporarily accepts syntax of both string types because the type of the string is undecided. Specifically, it accepts vertical whitespaces even if they are not allowed in single-line strings. The type of the string won’t be decided until the lexer sees a closing "# or a new line. In case of a closing "# where the string is single-line, the lexer will look back on the scanned characters for vertical whitespaces to decide if the single-line string is valid.
Using """ to start block string literals
This approach loses some convenience in using raw string literals while addressing the problem. For example, as discussed in issue #1359, #"""# is a natural way to write a string of ".
Non-quote marker after the open quote
Although something similar to C++ style like "( solves the problem, the syntax becomes complicated and hurts readability. In addition, "(")" is no simpler than "\"" or #"""#.
Use different quotes to allow #'"'#
When disallowing adjacent string literals, we can additionally allow [#]+' on single-line string literals. Another option is to use [#]+' for single-line string literals and [#]+" for block string literals. In general, #'"'# is visually confusing with character '"' and hurts readability.