Hi all
I am rather confused about default.
In section 4.3.4 the report says
"each ti must be a monotype for which Num ti holds"
but according to grep this is the only place "monotype" appears in the
report.
If I have the module
module TT (Foo(..)) where
default (Foo, [Foo], Foo -> Int)
data Foo = Foo deriving (Eq, Show)
instance Num Foo where {}
instance (Eq a, Show a) => Num [a] where {}
instance Eq (a -> b) where {}
instance Show (a -> b) where {}
instance Num (a -> b) where {}
then hugs accepts it and
TT> 5
Program error: Undefined member: fromInteger
ghc accepts it and
TT> 5
5
nhc tells me
Error when renaming::
Illegal type in default at 4:17
If I remove all the list and function stuff then hi says
TT> 5
5
If I have a
main = putStrLn $ show 5
then nhc gives me
TT> main
No default definition for class method fromInteger
and ghci gives me
Main> main
*** Exception: TT.lhs:8: No instance nor default method for class
operation PrelNum.fromInteger
It seems to me that hugs is in the right with it's handling of 5, but I
am not sure who is wrong with default ([Foo]) or (Foo -> Int).
Finally, the context free grammar doesn't currently enforce the
restriction that only one default declaration be given. Fixing it would
make rather a mess, though.
Ian
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