Pattern matching

[liquidev]

Having a way of matching against a set of destructuring patterns would make the language quite a bit more expressive.

Patterns

The following pattern types should be supported:

• _ - wildcard pattern. _ becomes a keyword and is used for discarding values in patterns.
• nil, true, 1, "abc" - literal patterns. For now only constant literals shall be supported
• abc - variable binding pattern. The variable name can be prefixed with mut to make the variable mutable (see Deep immutability #68)
• 1 or 2 - or-pattern, matches the left-hand side or the right-hand side

Additional patterns can be added as the language's feature set is expanded with tuples, lists, and dicts.

I also think constraints should be moved over to patterns, as that will allow for easier error checking in a lot of cases.

Assignment

Assignment shall use a pattern instead of an identifier on the left-hand side. If matching fails, a runtime error is produced.

1 = 1  # ok
a = 1  # ok
2 = nil  # error: pattern doesn't match
2 + 1 = 3  # error: invalid pattern - only literals, variables, and or is supported

Failing assignment does not always have to terminate execution, though. To allow for greater expressiveness, in the future we could make a special case for if and while where if the expression is an assignment, failure is considered falsy.

Because assignment is now a "match pattern or fail" operator, having it return a value wouldn't make much sense, so that functionality shall be dropped and nil should be produced instead of the old value stored in a variable.

match expression

A match expression should be introduced as a control flow construct to accompany if. match is comprised of a list of arms, with each arm being pattern -> expression. Arms are newline-separated.

x match
  p1 -> e1
  p2 -> e2
  p3 -> do
    x = 1  # do stuff
    e3
  end
  p4 -> e4
end

We use an infix match to better enable chaining multiple matches together.