Malcolm Wallace <[EMAIL PROTECTED]> wrote:
Just a comment, since a couple of people have made similar statements.
Haskell will derive Eq for arbitrarily complex types - there is no
restriction to "simple" types, whatever they might be.
Now that this topic is brought up...
Occasionally I would need to define recursive datatypes
using an explicit fixed-point operator, such as:
> data Fix f = In (f (Fix f)) deriving (Show, Eq)
> data L a x = Nil | Cons a x deriving (Show, Eq)
However, Haskell was not able to derive from Fix f any
instances. The following is what happens in GHCi:
*Main> In Nil == In Nil
Context reduction stack overflow; size = 21
Use -fcontext-stack20 to increase stack size to (e.g.) 20
`Eq (L e (Fix (L e)))' arising from use of `==' at <interactive>:1
`Eq (Fix (L e))' arising from use of `==' at <interactive>:1
<<deleted>>
*Main> In Nil
Context reduction stack overflow; size = 21
Use -fcontext-stack20 to increase stack size to (e.g.) 20
`Show (L e (Fix (L e)))'
arising from use of `print' at <interactive>:1
<<deleted>>
Probably Malcolm meant that Haskell will derive standard instances
for arbitrarily complex type **if they are definable manually**.
If so Malcolm's statement is true -- indeed I cannot even declare
the instances by hand. The following declaration is acceptable
by GHC after using the option -fallow-undecidable-instances,
but even though, I got the same error message as I try to print
In Nil.
> Instance Show (f (Fix f)) => Show (Fix f) where
> showsPrec _ (In x) = ("In ("++) . showsPrec 1 x . (')':)
This is rather unsatisfactory, because I would not be able
to inspect values of type Fix f in the interpreter. Is there
a way to get around this?
sincerely,
Shin
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