[Git][ghc/ghc][wip/or-pats] Write user guide entry

[David (@knothed)]

David pushed to branch wip/or-pats at Glasgow Haskell Compiler / GHC


Commits:
ad056dd6 by David Knothe at 2022-12-15T17:50:21+01:00
Write user guide entry

- - - - -


2 changed files:

- + docs/users_guide/exts/or_patterns.rst
- docs/users_guide/exts/patterns.rst


Changes:

=====================================
docs/users_guide/exts/or_patterns.rst
=====================================
@@ -0,0 +1,102 @@
+.. _or-patterns:
+
+Or-Patterns
+-------------
+
+.. extension:: OrPatterns
+    :shortdesc: Enable or-patterns.
+
+    :since: 9.8.1
+
+    Allow use of or-pattern syntax.
+
+Or-patterns are enabled by the language extension :extension:`OrPatterns`.
+
+They allow condensing multiple patterns into a single one. Suppose you had some sum type: ::
+
+    data Sweet = Cupcake | Liquorice | Cookie | Raisins
+    
+    tasty Cupcake = True
+    tasty Cookie = True
+    tasty _ = False
+
+All well and good, but what if we add another constructor to our type, like ``Cheesecake``?
+Because of the wildcard pattern we used when defining ``tasty``, the compiler doesn't warn us that the pattern match is incomplete,
+resulting in cheesecake incorrectly being characterised as untasty. What a shame!
+
+So if we want the compiler to aid us, we should have written out all cases explicitly, vertically bloating the code.
+Now or-patterns come in quite handy: With ``-XOrPatterns``, we can write: ::
+
+  tasty (one of Cupcake, Cookie) = True
+  tasty (one of Liquorice, Raisins) = False
+  
+If we extend ``Sweet`` by another constructor, we'll now get a warning about a non-exhaustive pattern match – given we compile with ``-Wincomplete-patterns``.
+
+While this may seem like a pointless example, it isn't: there are lots of places in GHC where constructor pattern matches either use a closing wildcard, or where all patterns are explicitly matched at the expense of code duplication. Just look at `Pat.hs <https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Hs/Pat.hs>`_.
+
+
+Specification
+~~~~~~~~~~~~~
+
+An or-pattern looks like this: ::
+
+  (one of pat_1, ..., pat_n)
+
+where ``pat_1``, ..., ``pat_n`` are patterns themselves. Or-Patterns are ordinary patterns and can be used wherever other patterns can be used.
+
+The result of matching a value ``x`` against this pattern is:
+
+- the result of matching ``x`` against ``pat_1`` if it is not a failure
+
+- the result of matching ``x`` against ``(one of pat_2, ..., pat_n)`` otherwise.
+
+
+The current main restriction on or-patterns is that **they may not bind any variables or constraints**. This prohibits code like ::
+
+  value :: Either a a -> a
+  value (one of Left x, Right x) = x -- binds a variable
+
+or ::
+
+  data G a where
+    G1 :: Num a => G a
+    G2 :: Num a => G a
+
+  bar :: G a -> a
+  bar (one of G1, G2) = 3 -- induces `Num a`
+
+  data GADT a where
+    IsInt1 :: GADT Int
+    IsInt2 :: GADT Int
+
+  foo :: a -> GADT a -> a
+  foo x (one of IsInt1 {}, IsInt2 {}) = x + 1 -- induces `a ~ Int`
+
+This is so because it is hard to specify good and correct static semantics for such or-patterns, but this could still be done sometime in the future.
+
+
+So what *can* or-patterns do?
+
+Apart from reducing code size and duplication, they interact with all forms of existing patterns, like view patterns and pattern synonyms: ::
+
+  f :: (Eq a, Show a) => a -> a -> Bool
+  f a (one of (== a) -> True, show -> "yes") = True
+  f _ _ = False
+
+  small (one of abs -> (one of 0, 1, 2), 3) = True -- -3 is not small
+  small _ = False
+
+  type Coll a = Either [a] (Set a)
+  pattern None <- (one of (Left []), Right (toList -> []))
+
+  empty None = False
+  empty _ = True
+
+Or-patterns do not employ backtracking when given guarded right hand sides, i.e. when one alternative of the or-pattern matches, the others are not tried when the guard fails. The following code yields ``"no backtracking"``: ::
+
+  case (True, error "backtracking") of
+    (one of (True, _), (_, True)) | False -> error "inaccessible"
+    _ -> error "no backtracking"
+
+
+(The exact syntax and semantics of or-patterns are found `here <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0522-or-patterns.rst#22static-semantics-of-or-pattern-matching>`_.)
\ No newline at end of file


=====================================
docs/users_guide/exts/patterns.rst
=====================================
@@ -10,3 +10,4 @@ Patterns
     view_patterns
     nk_patterns
     pattern_synonyms
+    or_patterns



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