[Haskell-cafe] MPTC and type classes issue (polymorphic '+')
Stefan O'Rear
stefanor at cox.net
Sat Apr 7 11:16:36 EDT 2007
On Sat, Apr 07, 2007 at 01:07:48PM +0100, Joel Reymont wrote:
> Folks,
>
> I'm trying to save time when typing in my ASTs so I thought I would
> create a Plus class like this (I do hide the one from Prelude)
>
> class PlusClass a b c | a b -> c where
> (+) :: a -> b -> c
>
> {-
> instance (Integral a, Integral b) => PlusClass a b Expr where
> a + b = NumExpr (NumOp Plus (Int (fromIntegral a)) (Int
> (fromIntegral b)))
>
> instance Integral a => PlusClass a Double Expr where
> a + b = NumExpr (NumOp Plus (Int (fromIntegral a)) (Double b))
> -}
>
> instance PlusClass Integer Integer Expr where
> a + b = NumExpr (NumOp Plus (Int a) (Int b))
Here is a large part of your problem - unsuprisingly enough, Integer
is an instance of Integral.
> instance PlusClass Double Integer Expr where
> a + b = NumExpr (NumOp Plus (Double a) (Int b))
>
> instance PlusClass NumExpr NumExpr Expr where
> a + b = NumExpr (NumOp Plus a b)
>
> instance PlusClass Integer NumExpr Expr where
> a + b = NumExpr (NumOp Plus (Int a) b)
>
> instance PlusClass NumExpr Integer Expr where
> a + b = NumExpr (NumOp Plus a (Int b))
>
> instance PlusClass String String Expr where
> a + b = StrExpr (StrOp StrPlus (Str a) (Str b))
>
> NumExpr and StrExpr return Expr whereas Int, Double return NumExpr
> and Str returns StrExpr.
You can probably use -fallow-incoherent-instances for this. It has a
scary name on purpose since it doesn't usually do what you think it
should... My (very limited!) understanding of type checking
algorithms says that in this case, the worst that can fail is an extra
call to fromInteger :: Integer -> Integer. Big deal.
> This is all so that I could type in
>
> input2 =
> [ InputDecs [ inp "emaLength" TyNumber (20 + 40) ] ]
>
> Still, I get the following error
>
> Easy/Test/ParserAST.hs:76:44:
> No instance for (PlusClass t t1 Expr)
> arising from use of `+' at Easy/Test/ParserAST.hs:76:44-50
> Possible fix: add an instance declaration for (PlusClass t t1 Expr)
> In the third argument of `inp', namely `(20 + 40)'
> In the expression: inp "emaLength" TyNumber (20 + 40)
> In the first argument of `InputDecs', namely
> `[inp "emaLength" TyNumber (20 + 40)]'
>
> and get an overlapped instances error if I uncomment the top portion.
Stefan
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