Phil writes:
| Joe writes:
|
| | Phil Writes:
| |
| | | The closest one can come is
| | |
| | | class G a where
| | | g :: a -> Bool
| | | g x = g [x]
| | |
| | | instance G Int
| | | instance G [Int]
| | | instance G [[Int]]
| | | ...
| | |
| | |which requires an infinite number of instance declarations.
| |
| | Can't this be written as follows?
| |
| | instance G Int
| | instance (G a) => G [a]
| |
| | Now, this is still an infinite number of instances, though not
| | declarations, so the point still holds that it can't be monomorphized.
|
| I don't think this works. The declaration `instance G Int' is valid
| only if `instance G [Int]' holds, and this in turn requires that
| `instance G [[Int]]' holds, and so on. I haven't tried it in Haskell,
| but it should either be illegal or cause the type-checker to enter an
| infinite loop. (Hmmm! Maybe someone should try it ...) Cheers, -- P
I forgot to mention that I did try it with hbc:
Silly.hs:
module Silly where
class G a where
g :: a -> Bool
g x = g [x]
instance G Int
instance (G a) => G [a]
Welcome to interactive Haskell B. version 0.998.5 SPARC 1992 Aug 27!
Loading prelude... 983 values, 70 types found.
Type "help;" to get help.
> load "Silly";
Loading "Silly.hs"
class G a where {
g :: a -> Bool
}
> let f x = g x
# ;
f :: (G a) => a -> Bool
Cheers,
--Joe
