Pattern matching

Quick examples:

case lhs, rhs of
| just(l), just(r) -> l ++ r
| just(l), nothing() -> l
| nothing(), just(r) -> r
| nothing(), nothing() -> []
end

Silver supports pattern matching on decorated nonterminal types. (It will also pattern match on undecorated nonterminal types, but implicitly “decorates” them with no inherited attributes.)

The expression has the following syntax:

case expressions... of
| patterns... -> expression
...
end

Patterns must be constructors of the appropriate types, the wildcard _, or a new name (a pattern variable) that will be bound to the value that appears in that position in the value being matched against.

Example:

local attribute val :: Maybe<Pair<String String>>;
-- ...
case val of
| just(pair(a, _)) -> "Key is " ++ a
| nothing() -> "Key not present"
end

produces a string value based on the value of val.

Pattern guards allow one to write more specific patterns that involve expressions. Guards can either be Boolean conditions or patterns. For example,

case val of
| just(x) when x > 0 -> x
| _ -> 42
end

is an example of a Boolean guard, where the first pattern will only match when the expression x > 0 evaluates to true.

An example of a pattern guard is

case name of
| just(n) when lookupBy(stringEq, n, top.env) matches just(ty) -> ty
| _ -> errorType()
end

Here the first pattern only matches when the value of the expression lookupBy(...) matches the pattern just(ty).

This page needs expanding and many of the old notes are now out of date.

• Looks-through-forwards
• Decoration
• Noting the types that can be matched on (int,string,list, any nonterminal type) and the syntax
• mention GADTs?