Algebraic effect constructs for Zilch

[Mesabloo]

• Reference issue: zilch-lang/gzc#7

To do:

• [ ] Describe the syntax for handlers
• [ ] Describe the syntax for effect declarations and applications
• [ ] Create small type inference rules only for effects

[Mesabloo]

The retained grammar for zilch effect constructs, as originally discussed in the linked discussion (at the bottom of the linked issue), is:

• effect handlers (potentially parameterized):

  forall-quantification ::= '<' (id+ ':' kind | id)*_',' ('|' type+_',')? '>'
  
  handler ::= 
  'handler' id forall-quantification? ('(' (id (':' type)?)*_',' ')')? ':=' { handler-branch+_; }
  handler-branch ::= 'return' id ('->' | '→') expr
                 | id '(' id*_, ')' ('->' | '→') expr

• effect declarations:

  effect ::= 'effect' id ('(' id*_',' ')')? ':=' { (id ':' type)*_; }

• effectful functions

  effect-row ::= '{' type*_',' ('|' id+_',')? '}'
  arrow-type ::= type-atom effect-row? ('->' | '→') type
             | '(' type+_',' ')' effect-row? ('->' | '→') type

  where type-atom is specifically defined not to include some types (e.g. arrow-type) and -> is equivalent to {}->

---

• x*_s is a shortcut for (x (s x)*)?
• x+_s is a shortcut for x (s x)*