Question

Given this:

data Foo = Bar { name :: String } | Baz { nickname :: String }

Both the functions name and nickname seem to be of type Foo -> String:

:t name
name :: Foo -> String

:t nickname
nickname :: Foo -> String

However, the definitions are incomplete since both of the following expressions will raise pattern match errors:

name $ Baz { nickname = "Bob" }
nickname $ Bar { name = "Fred" }

Is it possible to complete the definitions of name and nickname, i.e. something like:

name Baz { nickname = n } = ...

nickname Bar { name = n } = ...

Trying this in hugs yields errors like “Multiple declarations for variable name”.

Answer 1

No, it’s not possible. The field names are top-level functions with respect to scope and thus cannot be redefined or extended. It’s as impossible to make them total functions as it is to make head one.