I am pretty sure I have a good handle on type classes, but this one is perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3 (I know Haskell already has Tuples, just thought I would create my own for this exercise). Then I wanted to create a type class that would have a method called first that would get the first element of the tuple regardless of what kind of Tuple it is. Tuple2, Tuple3, etc. Here is what I have: data Tuple3 a b c = Tuple3 a b c deriving (Show) data Tuple2 a b = Tuple2 a b deriving (Show) class Indexable idx where first :: idx c -> a instance Indexable (Tuple2 a) where first (Tuple2 a b) = a In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One" I was greeted with the following trace: Couldn't match expected type ‘a1’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the instance declaration at TypeClasses.hs:35:10 ‘a1’ is a rigid type variable bound by the type signature for first :: Tuple2 a c -> a1 at TypeClasses.hs:36:4 Relevant bindings include a :: a (bound at TypeClasses.hs:36:18) first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4) In the expression: a In an equation for ‘first’: first (Tuple2 a b) = a Help is appreciated.
Quick adjustment, playing around too much with it, that should be: class Indexable idx where first :: idx a -> a Problem still exists. On Sun, Jan 24, 2016 at 11:25 PM, Daniel Hinojosa < dhinojosa@evolutionnext.com> wrote:
I am pretty sure I have a good handle on type classes, but this one is perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3 (I know Haskell already has Tuples, just thought I would create my own for this exercise). Then I wanted to create a type class that would have a method called first that would get the first element of the tuple regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.
Here is what I have:
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx where first :: idx c -> a
instance Indexable (Tuple2 a) where first (Tuple2 a b) = a
In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"
I was greeted with the following trace: Couldn't match expected type ‘a1’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the instance declaration at TypeClasses.hs:35:10 ‘a1’ is a rigid type variable bound by the type signature for first :: Tuple2 a c -> a1 at TypeClasses.hs:36:4 Relevant bindings include a :: a (bound at TypeClasses.hs:36:18) first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4) In the expression: a In an equation for ‘first’: first (Tuple2 a b) = a
Help is appreciated.
Got it to work this way but it got the wrong one. Still looking. instance Indexable (Tuple2 a) where first (Tuple2 b a) = a On Sun, Jan 24, 2016 at 11:39 PM, Daniel Hinojosa < dhinojosa@evolutionnext.com> wrote:
Quick adjustment, playing around too much with it, that should be:
class Indexable idx where first :: idx a -> a
Problem still exists.
On Sun, Jan 24, 2016 at 11:25 PM, Daniel Hinojosa < dhinojosa@evolutionnext.com> wrote:
I am pretty sure I have a good handle on type classes, but this one is perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3 (I know Haskell already has Tuples, just thought I would create my own for this exercise). Then I wanted to create a type class that would have a method called first that would get the first element of the tuple regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.
Here is what I have:
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx where first :: idx c -> a
instance Indexable (Tuple2 a) where first (Tuple2 a b) = a
In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"
I was greeted with the following trace: Couldn't match expected type ‘a1’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the instance declaration at TypeClasses.hs:35:10 ‘a1’ is a rigid type variable bound by the type signature for first :: Tuple2 a c -> a1 at TypeClasses.hs:36:4 Relevant bindings include a :: a (bound at TypeClasses.hs:36:18) first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4) In the expression: a In an equation for ‘first’: first (Tuple2 a b) = a
Help is appreciated.
Hello Daniel, it works with these tweaks: -- begin {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} module TupInst where data Tuple3 a b c = Tuple3 a b c deriving (Show) data Tuple2 a b = Tuple2 a b deriving (Show) class Indexable idx a where first :: idx -> a instance Indexable (Tuple2 a b) a where first (Tuple2 a0 b0) = a0 instance Indexable (Tuple3 a b c) a where first (Tuple3 a0 b0 c0) = a0 -- end call it in ghci like this: first $ Tuple3 (1::Int) 'a' False::Int
Ah. I had a suspicion it had to do with extensions. Got to study up on it.
Thanks
On Jan 25, 2016 12:42 AM, "Imants Cekusins"
Hello Daniel,
it works with these tweaks:
-- begin
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} module TupInst where
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx a where first :: idx -> a
instance Indexable (Tuple2 a b) a where first (Tuple2 a0 b0) = a0
instance Indexable (Tuple3 a b c) a where first (Tuple3 a0 b0 c0) = a0
-- end
call it in ghci like this:
first $ Tuple3 (1::Int) 'a' False::Int _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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Imants Cekusins