6.5.5. Record puns¶
-
NamedFieldPuns
¶ Since: 6.10.1 Allow use of record puns.
Record puns are enabled by the language extension NamedFieldPuns
.
When using records, it is common to write a pattern that binds a variable with the same name as a record field, such as:
data C = C {a :: Int}
f (C {a = a}) = a
Record punning permits the variable name to be elided, so one can simply write
f (C {a}) = a
to mean the same pattern as above. That is, in a record pattern, the
pattern a
expands into the pattern a = a
for the same name
a
.
Note that:
Record punning can also be used in an expression, writing, for example,
let a = 1 in C {a}
instead of
let a = 1 in C {a = a}
The expansion is purely syntactic, so the expanded right-hand side expression refers to the nearest enclosing variable that is spelled the same as the field name.
Puns and other patterns can be mixed in the same record:
data C = C {a :: Int, b :: Int} f (C {a, b = 4}) = a
Puns can be used wherever record patterns occur (e.g. in
let
bindings or at the top-level).A pun on a qualified field name is expanded by stripping off the module qualifier. For example:
f (C {M.a}) = a
means
f (M.C {M.a = a}) = a
(This is useful if the field selector
a
for constructorM.C
is only in scope in qualified form.)