6.5.4. Record field disambiguation

Implied by:RecordWildCards, DuplicateRecordFields

Allow the compiler to automatically choose between identically-named record fields (if the choice is unambiguous).

In record construction and record pattern matching it is entirely unambiguous which field is referred to, even if there are two different data types in scope with a common field name. For example:

module M where
  data S = MkS { x :: Int, y :: Bool }

module Foo where
  import M

  data T = MkT { x :: Int }

  ok1 (MkS { x = n }) = n+1   -- Unambiguous
  ok2 n = MkT { x = n+1 }     -- Unambiguous

  bad1 k = k { x = 3 }        -- Ambiguous
  bad2 k = x k                -- Ambiguous

Even though there are two x’s in scope, it is clear that the x in the pattern in the definition of ok1 can only mean the field x from type S. Similarly for the function ok2. However, in the record update in bad1 and the record selection in bad2 it is not clear which of the two types is intended.

Haskell 98 regards all four as ambiguous, but with the DisambiguateRecordFields extension, GHC will accept the former two. The rules are precisely the same as those for instance declarations in Haskell 98, where the method names on the left-hand side of the method bindings in an instance declaration refer unambiguously to the method of that class (provided they are in scope at all), even if there are other variables in scope with the same name. This reduces the clutter of qualified names when you import two records from different modules that use the same field name.

Since version 9.2.1, record fields in updates are disambiguated by ignoring non-field names in scope. For example, the following is accepted under DisambiguateRecordFields:

module Bar where
  import M  -- imports the field x

  x = ()

  e r = r { x = 0 }  -- unambiguously refers to the field

Some details:

  • Field disambiguation can be combined with punning (see Record puns). For example:

    module Foo where
      import M
      ok3 (MkS { x }) = x+1   -- Uses both disambiguation and punning
  • With DisambiguateRecordFields you can use unqualified field names even if the corresponding selector is only in scope qualified For example, assuming the same module M as in our earlier example, this is legal:

    module Foo where
      import qualified M    -- Note qualified
      ok4 (M.MkS { x = n }) = n+1   -- Unambiguous

    Since the constructor MkS is only in scope qualified, you must name it M.MkS, but the field x does not need to be qualified even though M.x is in scope but x is not (In effect, it is qualified by the constructor).