• simonpj's avatar
    [project @ 2005-10-14 11:22:41 by simonpj] · 36436bc6
    simonpj authored
    Add record syntax for GADTs
    Atrijus Tang wanted to add record syntax for GADTs and existential
    types, so he and I worked on it a bit at ICFP.  This commit is the
    result.  Now you can say
     data T a where
      T1 { x :: a }           	 :: T [a]
      T2 { x :: a, y :: Int } 	 :: T [a]
      forall b. Show b =>
     	T3 { naughty :: b, ok :: Int } :: T Int
      T4 :: Eq a => a -> b -> T (a,b)
    Here the constructors are declared using record syntax.
    Still to come after this commit:
      - User manual documentation
      - More regression tests
      - Some missing cases in the parser (e.g. T3 won't parse)
    Autrijus is going to do these.
    Here's a quick summary of the rules.  (Atrijus is going to write
    proper documentation shortly.)
    Defnition: a 'vanilla' constructor has a type of the form
    	forall a1..an. t1 -> ... -> tm -> T a1 ... an
    No existentials, no context, nothing.  A constructor declared with
    Haskell-98 syntax is vanilla by construction.  A constructor declared
    with GADT-style syntax is vanilla iff its type looks like the above.
    (In the latter case, the order of the type variables does not matter.)
    * You can mix record syntax and non-record syntax in a single decl
    * All constructors that share a common field 'x' must have the
      same result type (T [a] in the example).
    * You can use field names without restriction in record construction
      and record pattern matching.
    * Record *update* only works for data types that only have 'vanilla'
    * Consider the field 'naughty', which uses a type variable that does
      not appear in the result type ('b' in the example).  You can use the
      field 'naughty' in pattern matching and construction, but NO
      SELECTOR function is generated for 'naughty'.  [An attempt to use
      'naughty' as a selector function will elicit a helpful error
    * Data types declared in GADT syntax cannot have a context. So this
    is illegal:
    	data (Monad m) => T a where
    * Constructors in GADT syntax can have a context (t.g. T3, T4 above)
      and that context is stored in the constructor and made available
      when the constructor is pattern-matched on.  WARNING: not competely
      implemented yet, but that's the plan.
    Implementation notes
    - Data constructors (even vanilla ones) no longer share the type
      variables of their parent type constructor.
    - HsDecls.ConDecl has changed quite a bit
    - TyCons don't record the field labels and type any more (doesn't
      make sense for existential fields)
    - GlobalIdDetails records which selectors are 'naughty', and hence
      don't have real code.