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  1. {-# LANGUAGE KindSignatures, GADTs, TypeFamilies, TypeOperators, MultiParamTypeClasses, DataKinds #-}
  2.  
  3. import Control.Applicative
  4. import Data.Traversable (Traversable)
  5.  
  6. class (Applicative f, Traversable f) => Dim f
  7.  
  8. class Shapely (fs :: [* -> *])
  9. instance Shapely '[]
  10. instance (Dim f, Shapely fs) => Shapely (f ': fs)
  11.  
  12. -------------------------------------
  13. --            Hypercuboid datatype
  14. -------------------------------------
  15. data Hyper :: [* -> *] -> * -> * where
  16.     Scalar :: a -> Hyper '[] a
  17.     Prism  :: (Dim f, Shapely fs) => 
  18.                 Hyper fs (f a) -> Hyper (f ': fs) a
  19.  
  20. instance Functor (Hyper fs) where
  21.     fmap f (Scalar a) = Scalar $ f a
  22.     fmap f (Prism p)  = Prism $ fmap (fmap f) p
  23.  
  24. instance Applicative (Hyper fs) where
  25.     pure a = undefined 
  26. {- `pure a = Scalar a` gives:
  27.             Couldn't match type ‘fs’ with ‘'[]’
  28.                   ‘fs’ is a rigid type variable bound by
  29.                        the instance declaration
  30.             Expected type: Hyper fs a
  31.               Actual type: Hyper '[] a
  32. -}
  33.  
  34. main = print 42
Success #stdin #stdout 0s 4700KB
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Standard input is empty
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42
https://ideone.com/9JnzJD
language:
Haskell (ghc 8.4.4)
created:
9 years ago
visibility:
public

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