Mini-SIP for case if (#1076)

Author: som-snytt

_sips/sips/2018-05-26-case-if.md

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+---
+layout: sips
+discourse: true
+title: SIP-NN - Uncluttering Abuse of Match
+
+vote-status: new
+vote-text:
+permalink: /sips/:title.html
+redirect_from: /sips/pending/case-if.html
+
+---
+
+**By: Som Snytt and A. P. Marki**
+
+## History
+
+| Date          | Version       |
+|---------------|---------------|
+| May 26th 2018 | Initial Draft |
+
+## Motivation
+
+Since the demise of SIP-12, we have relied on Marie Kondo to declutter
+what remains of our lives. But we can do better.
+
+Currently, `case` syntax requires an underscore to represent a pattern,
+even if the code of interest is the guard that follows.
+
+Anxiety over this underscore is expressed in [a StackOverflow question][1]
+in which a programmer mulls the question of "Abuse of Match" and whether
+it actually makes you go blind.
+
+We propose to go underscoreless, pace the famous consultancy.
+
+If we can't have SIP-12 then we can have tidy syntax for `if-then` in `case` blocks.
+
+## Syntax
+
+In lieu of
+
+    42 match {
+      case _ if now isAfter midnight => nothingGoodHappens()
+    }
+
+we write
+
+    42 match {
+      case if now isAfter midnight => nothingGoodHappens()
+    }
+
+The syntax accepts either a pattern with optional guard, or a guard with no pattern:
+
+    CaseClause        ::=  ‘case’ (Pattern [Guard] | Guard) ‘=>’ Block
+    Guard             ::=  ‘if’ PostfixExpr
+
+A guard with no pattern is taken as though the pattern were an underscore, which matches
+any value.
+
+This modest proposal eschews dispensing with the pattern altogether, `case =>`.
+
+## Further Justifications
+
+In a respected online [forum][2] which brooks no fools, [Mulliganaceous][3] has posted
+an "accepted" answer using the idiom, "in case if". This supports `case if` as
+a natural locution.
+
+In a response to one judgment about abuse of match, [one Scala user][4] whose handle I can
+never spell quite right let alone pronounce finds the match version
+"clearer and visually more pleasant" than the cluttered `if` expression.
+
+From the beginning, Scala has made great strides in reducing vertical space in source code.
+However, we are still constrained horizontally, despite curved OLED screens.
+Recently, [a suggested edit][5] was declined because of maximum line length restrictions.
+Every wasted character brings us closer to an unfortunate line break.
+
+## Implementation
+
+An implementation is [available][6]. It's pretty slick.
+
+## References
+
+1. [Abuse of Match?][1]
+2. [Reputation requirements for creating tags and tag synonyms][2]
+3. [Mulliganaceous user profile][3]
+4. [huynhjl user profile][4]
+5. [Sample line length limitation in a Scala project][5]
+6. [Implementation][6]
+
+[1]: https://stackoverflow.com/questions/12556236/abuse-of-match "Abuse of Match?"
+[2]: https://meta.stackoverflow.com/a/368537/1296806 "Reputation requirements for creating tags and tag synonyms"
+[3]: https://meta.stackoverflow.com/users/8242447/mulliganaceous "Mulliganaceous"
+[4]: https://stackoverflow.com/users/257449/huynhjl "huynhjl"
+[5]: https://github.com/apache/spark/pull/21369/files#r189794046 "scala-style enforces a max of 100 chars per line"
+[6]: https://github.com/scala/scala/pull/6241 "Implementation PR 6241"
+