6.5.7. Record puns

NamedFieldPuns

Since:

6.10.1

Status:

Included in GHC2024, GHC2021

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 constructor M.C is only in scope in qualified form.)