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followup strformat PR. backslash escapes, tests, docs (#17700)

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* Allow use of colons inside fmt
allowing colons inside fmt by replacing the format specifier delimiter lets arbitrary nim code be run within fmt expressions.

Co-authored-by: flywind <xzsflywind@gmail.com>

* formatting,documentation,backslash escapes

Adding support for evaluating expressions by special-casing parentheses causes this regression: `&"""{ "(hello)" }"""` no longer parses.
In addition, code such as &"""{(if open: '(' else: ')')}""" wouldn't work.
To enable that, as well as the use of, e.g. Table constructors inside curlies, I've added backslash escapes.
This also means that if/for/etc statements, unparenthesized, will work, if the colons are escaped, but i've left that under-documented.

It's not exactly elegant having two types of escape, but I believe it's the least bad option.

* changelog
* added json strformat test
* pulled my thumb out and wrote a parser

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: flywind <xzsflywind@gmail.com>
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shirleyquirk 2021-04-12 06:32:37 +01:00 • committed by GitHub
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3 changed files with 89 additions and 5 deletions
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33
lib/pure/strformat.nim
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@ -144,6 +144,19 @@ An expression like `&"{key} is {value:arg} {{z}}"` is transformed into:
Parts of the string that are enclosed in the curly braces are interpreted
as Nim code, to escape a `{` or `}`, double it.
Within a curly expression,however, '{','}', must be escaped with a backslash.
To enable evaluating Nim expressions within curlies, inside parentheses
colons do not need to be escaped.
]##
runnableExamples:
let x = "hello"
assert fmt"""{ "\{(" & x & ")\}" }""" == "{(hello)}"
assert fmt"""{{({ x })}}""" == "{(hello)}"
assert fmt"""{ $(\{x:1,"world":2\}) }""" == """[("hello", 1), ("world", 2)]"""
##[
`&` delegates most of the work to an open overloaded set
of `formatValue` procs. The required signature for a type `T` that supports
formatting is usually `proc formatValue(result: var string; x: T; specifier: string)`.
@ -289,6 +302,7 @@ keep the hygiene of `myTemplate`, and we do not want `arg1`
to be injected into the context where `myTemplate` is expanded,
everything is wrapped in a `block`.
# Future directions
A curly expression with commas in it like `{x, argA, argB}` could be
@ -588,10 +602,21 @@ proc strformatImpl(pattern: NimNode; openChar, closeChar: char): NimNode =
var subexpr = ""
var inParens = 0
while i < f.len and f[i] != closeChar and (f[i] != ':' or inParens!=0):
var inSingleQuotes = false
var inDoubleQuotes = false
template notEscaped:bool = f[i-1]!='\\'
while i < f.len and f[i] != closeChar and (f[i] != ':' or inParens != 0):
case f[i]
of '(': inc inParens
of ')': dec inParens
of '\\':
if i < f.len-1 and f[i+1] in {openChar,closeChar,':'}: inc i
of '\'':
if not inDoubleQuotes and notEscaped: inSingleQuotes = not inSingleQuotes
of '\"':
if notEscaped: inDoubleQuotes = not inDoubleQuotes
of '(':
if not (inSingleQuotes or inDoubleQuotes): inc inParens
of ')':
if not (inSingleQuotes or inDoubleQuotes): dec inParens
of '=':
let start = i
inc i
@ -604,7 +629,7 @@ proc strformatImpl(pattern: NimNode; openChar, closeChar: char): NimNode =
else: discard
subexpr.add f[i]
inc i
var x: NimNode
try:
x = parseExpr(subexpr)