Difference between revisions of "Name clashes in record fields"

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(Using language extension: fix documentation link for duplicate record fields)
(link to latest released user guide, not master)
 
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=== Using [[language extension]] ===
 
=== Using [[language extension]] ===
   
<code>[https://ghc.gitlab.haskell.org/ghc/doc/users_guide/glasgow_exts.html#duplicate-record-fields DuplicateRecordFields]</code> extension (GHC 8.0.1+) allow definition of record types with identically-named fields.
+
<code>[https://downloads.haskell.org/ghc/latest/docs/html/users_guide/glasgow_exts.html#duplicate-record-fields DuplicateRecordFields]</code> extension (GHC 8.0.1+) allow definition of record types with identically-named fields.
   
 
== See also ==
 
== See also ==

Latest revision as of 19:41, 4 April 2019

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

DuplicateRecordFields extension (GHC 8.0.1+) allow definition of record types with identically-named fields.

See also