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sips/pending/_posts/2015-08-08-intuitive-interpolation-escapes.md

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+---
+layout: sip
+disqus: true
+title: SIP-26 - Intuitive Interpolation Escapes
+---
+
+__A. P. Marki__
+
+__first submitted 8 August 2015__
+
+## Motivation ##
+
+String interpolation is a popular feature. Everyone wants to write s"hello, \"$world\"" but they can't.
+
+Currently, the parser takes an interpolated string as arbitrary characters between double quotes (`"`).
+
+Interpolated arguments are introduced by dollar sign (`$`) and the dollar sign itself can be escaped by
+doubling it (`$$`).
+
+The current parser is agnostic about backslash as an escape, and it is up to the interpolator to
+determine the semantics of backslash escapes and whether escapes are supported at all.
+
+Literal double quote is supported by interpolated expressions:
+
+    s"hello, ${ '"' }$world${ '"' }"
+
+It has been informally proposed to support literal quote with a dollar escape:
+
+    s"hello, $"$world$""
+
+A formatting interpolator can define its own convention:
+
+    F"hello, %q$world%s%q"
+
+These alternatives are not as intuitive as the standard escape for double quote as supported for
+string literals.
+
+## Syntax ##
+
+The proposed syntax is that the parser will honor backslash escapes for `$`, `"` and `\`.
+
+Since the parser does not translate any escapes, the semantics of these escapes are defined by
+the interpolator.
+
+    s"\$s to \"do\\nuts\""
+
+can be expressed currently:
+
+    s"$$s to ${'"'}do\\nuts${'"'}"
+
+The backslash is escaped because the `s`-interpolator processes standard escapes.
+
+Under this proposal, `\$` is also processed by the `s`-interpolator.
+
+For uniformity, dollar escapes are proposed for each of `$$`, `$"` and `$\`.
+
+    raw"$\$$"     // "\\$"
+
+This is the same as
+
+    raw"\$"
+
+since the parser passes the escaped text `\$` to the `raw`-interpolator, which ignores escapes.
+
+But because the parser will honor the escaped dollar, and the `raw`-interpolator abjures all escapes,
+
+    val x = "X"
+    raw"\$x"        // "\$x"
+    raw"$\$x"       // "\X"
+
+To support trailing backslash in the interpolated string, an escaped double quote is taken as
+a literal backslash and terminal quote if it is the last double quote on the line.
+
+    s"C:\Program Files\"
+
+More precisely, the parser will locate the end of an ordinary (not triple-quoted) interpolated string
+by finding the next unescaped quote (or the last escaped quote on the line if all remaining quotes are escaped).
+
+### Examples
+
+{% highlight scala %}
+    val x = "X"
+    val rx =r"\Q$x\E\\\$"
+    "X\\" match { case rx() => true }
+{% endhighlight %}
+
+## Restrictions ##
+
+Previously,
+
+  raw"hello, \" contains "hell"
+
+would return true due to quoting hell.
+
+Under this proposal, the third quote character would terminate the interpolated string and the remaining text
+would not parse because the last quote starts a string literal that is not terminated.
+
+## See Also ##
+