Name clashes in record fields

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Question

error| Multiple declarations of ‘xxx’

I like to define:

data Human = Human {name :: String}
data Dog = Dog {name :: String}

Why is this forbidden?

I like to define:

data Human = Human {name :: String}

name :: Cat -> String
name = ...

Why is this forbidden, too?

Answer

The record field accessors name are just functions that retrieve the field's value from a particular record. They are in the global scope together with top-level functions and thus cannot have the same name. For resolving this you may:

  • rename the accessor or the top-level function
  • put the data declaration or the top-level function in another module and import qualified
  • write a typeclass with a name function and fit the non-accessor function name somehow into that.

Using language extension

The GHC Users Guide has a DuplicateRecordFields section.. The extension (GHC 8.0.1+) allow definition of record types with identically-named fields.

See also