this works: {-# LANGUAGE MultiParamTypeClasses,FlexibleInstances #-} module Lift where newtype MyNumbers a = MyNum a deriving Show class Numbers a where sqr :: a -> a class Lifts b a where lift0 :: a -> b a lift1 :: (a -> a) -> b a -> b a instance Lifts MyNumbers a where lift0 x = MyNum x lift1 o (MyNum x) = MyNum (o x) instance Numbers Float where sqr x = x * x instance Numbers (MyNumbers Float) where sqr x = lift1 sqr x ​

So it seems that (1) lift1 o x = MyNum (o x) Needed to be rewritten to (2) lift1 o (MyNum x) = MyNum (o x) I am not sure why the constructor is used on the LHS of (2) Thanks for all your help. Pat On 2 October 2016 at 17:03, PATRICK BROWNE wrote:

Yes, I am running ghci 8.0.1. Below, I have added the language pragmas suggested by the compiler Regards, Pat . {-# LANGUAGE MultiParamTypeClasses,FlexibleInstances,FlexibleContexts,UndecidableInstances #-}

data MyNumbers a = MyNum a deriving Show

class (Floating a) => Numbers a where sqr :: a -> a

class Lifts b a where lift0 :: a -> b a lift1 :: (a -> a) -> b a -> b a

instance Lifts MyNumbers a where lift0 x = MyNum x lift1 o x = MyNum (o x)

instance (Floating (MyNumbers Float)) => Numbers (MyNumbers Float) where sqr x = lift1 sqr x

On 2 October 2016 at 16:38, Francesco Ariis wrote:

...

On Sun, Oct 02, 2016 at 03:54:07PM +0100, PATRICK BROWNE wrote:

...

Hi, I am trying, without sucess, to run the code below from [1] (Section 4.3). Obviouly I am missing something. I would like to keep as close to the original code as possible. Any help would be apreciated. Regards, Pat [1] http://publik.tuwien.ac.at/files/pub-geo_2321.pdf

I get different errors from yours (missing extension, Illegal instance declaration). Can you paste the exact file you are loading with ghci (because you are using ghci, right?)? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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