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Discuss (*) patterns in reference documentation

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Tim Chevalier 2012-04-23 15:50:16 -07:00
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@ -2155,10 +2155,30 @@ alt_pat : pat [ "to" pat ] ? [ "if" expr ] ;
An `alt` expression branches on a *pattern*. The exact form of matching that
occurs depends on the pattern. Patterns consist of some combination of
literals, destructured enum constructors, records and tuples, variable binding
specifications and placeholders (`_`). An `alt` expression has a *head
specifications, wildcards (`*`), and placeholders (`_`). An `alt` expression has a *head
expression*, which is the value to compare to the patterns. The type of the
patterns must equal the type of the head expression.
In a pattern whose head expression has an `enum` type, a placeholder (`_`) stands for a
*single* data field, whereas a wildcard `*` stands for *all* the fields of a particular
variant. For example:
~~~~
enum list<X> { nil, cons(X, @list<X>) }
let x: list<int> = cons(10, @cons(11, @nil));
alt x {
cons(_, @nil) { fail "singleton list"; }
cons(*) { ret; }
nil { fail "empty list"; }
}
~~~~
The first pattern matches lists constructed by applying `cons` to any head value, and a
tail value of `@nil`. The second pattern matches `any` list constructed with `cons`,
ignoring the values of its arguments.
To execute an `alt` expression, first the head expression is evaluated, then
its value is sequentially compared to the patterns in the arms until a match
is found. The first arm with a matching pattern is chosen as the branch target