On 8 Apr 2008, at 17:03, Christian Maeder wrote:
"deriving Eq" i.e. following "data List a = List a" creates an
instance
like:
instance Eq a => Eq (List a) where <compiler implementation>
The problem was discussed for Stand-alone deriving declarations:
http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html
There is a difference between
data Foo a = Bar a | Baz String
deriving instance Eq a => Eq (Foo a)
and
data Eq a => Foo a = Bar a | Baz String deriving (Eq)
The second is more restrictive, as it requires Eq for the
construction of Foo a, whereas the first could be construed to mean
that Eq a is only required for the construction of Eq(Foo a). So the
first could be used without Eq a if Eq(Foo a) is not needed. This is
good when constructing libraries for general types.
Hans
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