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