This pull request features a full overhaul of the pattern matching expression

[EliasC]

It:

• Adds guards.
• Adds support for matching on objects via Scala-like extractor methods.
• Adds tuples and matching on tuples. However, at the moment the only way to get a value out of a tuple is via pattern matching.
• Adds documentation to the match expression.

Tuples are necessary in order to do anything interesting with extractor methods which is why they are included in the same pull request.

It also fixes some related issues:

• Deep compiler error bug — "encorec: Type.hs: no encoreArgTTag for encore_arg_t" #265
• Match invents numbers with non-total pattern #253

Sample code implementing a polymorphic association list using pattern matching in the lookup method:

def strCmp(s1 :string, s2 :string) :int
  embed int
    strcmp(#{s1}, #{s2});
  end

passive class Link<t>
  value :t
  next :Link<t>

  def init(value :t, next :Link<t>) :void {
    this.value = value;
    this.next = next;
    }

  -- Methods with the type void -> Maybe T are extractor methods. They define
  -- new patterns that may be used to match against objects of this class
  def link() : Maybe(t, Link<t>)
    Just (this.value, this.next)

passive class AssocList<kt, vt>
  first :Link<(kt, vt)>
  cmp :(kt, kt) -> int

  def init(cmp :(kt, kt) -> int) :void {
    this.first = null;
    this.cmp = cmp
    }

  def put(key :kt, value :vt) :void {
    this.first = new Link<(kt, vt)>((key, value), this.first)
    }

  def lookup(key :kt) : Maybe vt {
    this.lookupHelper(key, this.first, this.cmp)
    }

  def lookupHelper(key :kt, current :Link<(kt, vt)>, cmp :(kt, kt) -> int) :Maybe vt{
    match current with
      null => Nothing : Maybe vt
      link((k, v), next) when cmp(k, key) == 0 => Just v
      link(x, next) => this.lookupHelper(key, next, cmp)
      }

class Main
  def main() : void
    let l = new AssocList<string, int>(strCmp)
    in{
      l.put("foo", 42);
      l.put("bar", 4711);
      match l.lookup("bar") with
        Just res => print res
        Nothing => print "Error"
    }

[EliasC]

@glundi: Note that you are the author of the commit, even though I created the PR (your contributions will still go into history 😃)

[ghost]

Thanks! Should I close the other PR?

[EliasC]

We have a meeting tomorrow, after which we should have decided which branch this goes into.