Question

I just ran across the following syntax in a piece of Haskell code –

data A = A Int Int | B

m :: A -> Int
m a = case a of
  A{} -> 1
  _ -> 2

What is the A{} doing here? Does the {} automatically match for any number of arguments?

I have a feeling that this is exploiting the fact that Haskell record syntax desugars to a bunch of functions and a regular Algebraic Datatype. Is that the case?

Answer 1

Yes, A{} matches any value constructed with the A constructor, regardless of whether the type has been declared with record syntax or not.

The language report specifies

The expression F {}, where F is a data constructor, is legal whether or not F was declared with record syntax (provided F has no strict fields — see the fourth bullet above); it denotes F ⊥1 … ⊥n, where n is the arity of F.

The ‘fourth bullet’ mentioned in the parenthesis states that it is a static error to construct a value with record syntax which omits a strict field.

And in the section on pattern matching, one of the grammar rules for patterns is

apat -> qcon { fpat1 , … , fpatk }      (labeled pattern, k ≥ 0)

and the semantics are given in the subsection on formal semantics of pattern-matching (3.17.3) as

(o) case  v  of {  K  {} ->  e ; _ ->  e′ }
        = case  v  of {
            K _… _ ->  e ; _ ->  e′ }