6.5.3. Record field disambiguation¶
- DisambiguateRecordFields¶
- Since:
- 6.8.1 
 - Allow the compiler to automatically choose between identically-named record selectors based on type (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.
Some details:
- Field disambiguation can be combined with punning (see Record puns). For example: - module Foo where import M x=True ok3 (MkS { x }) = x+1 -- Uses both disambiguation and punning 
- With - DisambiguateRecordFieldsyou can use unqualified field names even if the corresponding selector is only in scope qualified For example, assuming the same module- Mas 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 - MkSis only in scope qualified, you must name it- M.MkS, but the field- xdoes not need to be qualified even though- M.xis in scope but- xis not (In effect, it is qualified by the constructor).