CODEWITHSUNDEEP
julia
Cameron Bieganek
Cameron Bieganek

Reputation: 7674

How does a non-standard string literal avoid a syntax error generated by a standard string literal?

Based on the relevant section of the Julia docs, my understanding is that a non-standard string literal like foo"hello, world" is equivalent to explicitly calling the corresponding macro: @foo_str("hello, world"). However, there must be some extra magic that I'm not understanding. Consider a date format dateformat"\m". By itself, "\m" throws a syntax error:

julia> "\m"
ERROR: syntax: invalid escape sequence

And the same syntax error is thrown if I call @dateformat_str("\m"), since the string literal "\m" appears to be evaluated or error checked before it is passed to the macro:

julia> using Dates

julia> @dateformat_str("\m")
ERROR: syntax: invalid escape sequence

However, using the non-standard string literal works:

julia> dateformat"\m"
dateformat"\m"

This is counter-intuitive, because I thought that dateformat"\m" was equivalent to @dateformat_str("\m"). How does the non-standard string literal avoid the syntax error generated by the standard string literal?

Upvotes: 8

Views: 367

Answers (1)

Frames Catherine White
Frames Catherine White

Reputation: 28212

In short, because the parser recognizes that situation and parses the string literal differently

For string macros invocations it does this. Calling: parse-raw-literal

Where as for normal string literals it does this. Calling parse-string-literal

@dateformat_str("\m") on the other hand parses as a macro invocation on a normal string literal. so it uses the later parse-string-literal, which errors.

Note that ones parsed it has parsed the string into what is escaped as "\\m" 

julia> dump(:(dateformat"\m"))
Expr
  head: Symbol macrocall
  args: Array{Any}((3,))
    1: Symbol @dateformat_str
    2: LineNumberNode
      line: Int64 1
      file: Symbol REPL[6]
    3: String "\\m

Of related interest is the raw string macro , which does nothing at all, but is still going to be string parsed using parse-raw-literal It is basically defined as

macro raw_str(s)
    return s
end

Upvotes: 8

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