Name clashes in record fields: Difference between revisions
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== Question == | == Question == | ||
<code> error| Multiple declarations of ‘xxx’ </code> | |||
I like to define: | I like to define: | ||
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data Dog = Dog {name :: String} | data Dog = Dog {name :: String} | ||
</haskell> | </haskell> | ||
Why is this forbidden? | |||
Why is this forbidden? | |||
I like to define: | I like to define: | ||
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* write a typeclass with a <hask>name</hask> function and fit the non-accessor function <hask>name</hask> somehow into that. | * write a typeclass with a <hask>name</hask> function and fit the non-accessor function <hask>name</hask> somehow into that. | ||
=== Using [[language extension]] === | |||
{{GHCUsersGuide|exts/duplicate_record_fields||a DuplicateRecordFields section}}. The extension (GHC 8.0.1+) allow definition of record types with identically-named fields. | |||
== See also == | == See also == | ||
* [ | * [https://gitlab.haskell.org/haskell/prime/-/wikis/type-directed-name-resolution ''Type directed name resolution'' in the Haskell Prime Wiki] | ||
* Haskell-Cafe on [http://www.haskell.org/pipermail/haskell-cafe/2010-December/087859.html Record types and unique names] | * Haskell-Cafe on [http://www.haskell.org/pipermail/haskell-cafe/2010-December/087859.html Record types and unique names] | ||
[[Category:FAQ]] | [[Category:FAQ]] | ||
Latest revision as of 20:08, 23 July 2021
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
namefunction and fit the non-accessor functionnamesomehow 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.