\" escape does not work with string interpolation

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scala> val a="42"
a: String = 42

scala> "a=\"$a\""
res33: String = a="$a"    // ok

scala> s"a=\"$a\""
<console>:1: error: ';' expected but string literal found.
       s"a=\"$a\""
                ^         // oops

scala> s"a=\'$a\'"
res34: String = a='42'    // ok with \'

scala> """a=\"$a\""""
res35: String = a=\"$a\"  // escape not interpreted, ok

scala> s"""a=\"$a\""""
res36: String = a="42"    // escape interpreted, should it ?

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Imported From: https://issues.scala-lang.org/browse/SI-6476?orig=1
Reporter: Denis Petrov (denis)
Affected Versions: 2.10.0-M6

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@paulp said:
You can see this being declared wontfix here: https://groups.google.com/d/topic/scala-sips/d2K23f__6b0/discussion

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Denis Petrov (denis) said (edited on Oct 4, 2012 3:24:35 PM UTC):
That's bad.

Without interpolation, single quote literals could represent any string using escapes (triple-quoted cannot represent """)
With interpolation enabled, there are still literals which cannot be represented using both single- and triple- quoted sequences.

Thus, a code generator (i.e. template engine) cannot use triple quoted in the emitted code, as literals are not known in advance.
It can use single quotes, though, and escape any literal into them.

But as result of this escaping rules, a code generator cannot use string interpolation at all.

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Denis Petrov (denis) said:
As '$' is the escape symbol in interpolated strings (think s"$$" == "$"), it may have sense to escape '"' also via $-prefixed escape sequence (say s"$"" == """).

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@retronym said:
Closing as "Won't fix" as the behaviour is in accordance with the spec.

I know it's a little clunky, but your code generator could create:

scala> s"${'"'}"
res1: String = "

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Denis Petrov (denis) said:
Hopefully, s"\042" works (but s"\u0022" does not).

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Radim Kolar (hsn) said:
Its realistic to get spec updated and this issue fixed?

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@jrudolph said:
I'll try to summarize the case in other words: Scala doesn't want to make any choices about escaping in interpolated strings. Especially, backslash is should be freed as escaping character on the scala grammar level. This means that \" can't work because it would mean that backslash itself would need to be escaped.

I agree to what Denis said: as there is a new escape character $ in interpolated strings, it would make sense to escape double-quotes with $". Any counter-arguments?

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@retronym said:
+1 to s"$"". Because it doesn't compile today, we don't risk changing the meaning of existing programs.

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@densh said:
Non-working escaping bugged me a bit before I got used to """. I guess this is quite logical to let people escape quotes in their string interpolators. e.g.

q"method.call($"mystring in a quasiquote$")"

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@retronym said (edited on Sep 19, 2013 4:03:44 PM UTC):
WIP:

Update Scala parser:

retronym/scala@scala:master...ticket/6476

Update IntelliJ parser:

https://github.com/retronym/intellij-scala/compare/topic/SI-6476

Starting a [scala-internals] discussion: https://groups.google.com/d/msg/scala-internals/_DnZGO-4NXc/xNoiOUoCf58J

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Jason Swartz (swartzrock) said:
Re comments from Martin and others here make it appear that string interpolation should not interpret the backspace:
https://groups.google.com/forum/#!topic/scala-sips/d2K23f__6b0/discussion

A triple-quoted string interpolation should preserve the backspace.
s"""a="$a""""
expected result: a="42"
actual resultl: a="42"

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@som-snytt said:
The language is that the parser preserves the escapes in an interpolated string, so that the interpolator can do whatever it wants.

Clarifying the previous comment:

scala> implicit class `literally the string`(val sc: StringContext) {
     | def lit(args: Any*): String = {
     |   sc.checkLengths(args)
     |   sc.standardInterpolator(identity, args)
     | }}
defined class literally$u0020the$u0020string

scala> val a = "42" ; s"""a=\"$a\";"""
a: String = 42
res4: String = a="42";

scala> val a = "42" ; lit"""a=\"$a\";"""
a: String = 42
res5: String = a=\"42\";

Clarifying a previous suggestion to use octal escapes, they are deprecated somewhat:

scala> implicit class `strictly ess`(val sc: StringContext) {
     | def S(args: Any*): String = {
     |   sc.checkLengths(args)
     |   sc.standardInterpolator(StringContext.processEscapes, args)
     | }}
defined class strictly$u0020ess

scala> val a = "42" ; s"\042$a\042"
a: String = 42
res0: String = "42"

scala> val a = "42" ; S"\042$a\042"
scala.StringContext$InvalidEscapeException: invalid escape character at index 0 in "\042"
  at scala.StringContext$.treatEscapes0(StringContext.scala:208)
  at scala.StringContext$.processEscapes(StringContext.scala:189)
  at strictly$u0020ess$$anonfun$S$1.apply(<console>:10)
  at strictly$u0020ess$$anonfun$S$1.apply(<console>:10)
  at scala.StringContext.standardInterpolator(StringContext.scala:121)
  at strictly$u0020ess.S(<console>:10)
  ... 32 elided

scala> :se +deprecation

scala> val a = "42" ; f"\042$a\042"
<console>:11: warning: Octal escape literals are deprecated, use \u0022 instead.

And of course as noted previously, that advice is only good for triple-quotes. There's a comment in the code to upgrade the advice to dollar-quote when that is merged someday.

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@som-snytt said (edited on Jul 9, 2014 5:12:18 AM UTC):
I got antsy, so @retronym's patch is on scala/scala#3870, which is under the aegis of SIP-24.

https://groups.google.com/d/msg/scala-sips/rem6D3XTkPY/08v2X0ZpmgsJ

The literal quote is what @retronym would call the least controversial bit.

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@som-snytt said:
I defected and proposed

scala/scala#4308

and I never understood the confusion over this topic.

I find the arguments against the interpolator receiving the raw text unconvincing. (That escapes must be processed at one layer as meta-escapes and so on.)

Everyone wants to say s"\"hello,\" world".

I think the compiler should satisfy that customer.