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
DisambiguateRecordFields
you can use unqualified field names even if the corresponding selector is only in scope qualified For example, assuming the same moduleM
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 itM.MkS
, but the fieldx
does not need to be qualified even thoughM.x
is in scope butx
is not (In effect, it is qualified by the constructor).