Hi!
Yes, I know, the following code snippets are wrong, because instances are
always imported. The compiler yields an error message, like it should:
Foo1.lhs:2:
Duplicate or overlapping instance declarations
for `Read FooData'
Name.lhs:382: Non-exhaustive patterns in function provSrcLoc
But the "Non-exhaustive patterns" error shouldn't be there, should it?
BTW: is it possible to show the names of the overlapping modules?
Cheers,
Michael
\begin{code}
module Data where
data FooData = FooData
\end{code}
\begin{code}
module Foo2 where
import Data
instance Read FooData where
readsPrec _ _ = [(FooData,"")]
\end{code}
\begin{code}
module Foo3 where
import Data
instance Read FooData where
readsPrec _ _ = [(FooData,"")]
\end{code}
\begin{code}
module Foo1 where
import Data
import Foo2
import Foo3
data Baz = Baz deriving Read -- if this line is commented out -> no error
trigger = FooData -- else, if this line is commented out -> no error
-- instances only imported if used?
\end{code}
--
XXXVII:
Ninety percent of the time things will turn out worse than you expect.
The other 10 percent of the time you had no right to expect so much.
