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| author | Simon Peyton Jones <simonpj@microsoft.com> | 2011-05-09 11:53:47 +0100 |
|---|---|---|
| committer | Simon Peyton Jones <simonpj@microsoft.com> | 2011-05-09 11:53:47 +0100 |
| commit | cbebca1c9164a5e5ae9b117d0dcf5ad217defc6d (patch) | |
| tree | 1a02513eeb14c54d1f538fb580cebc232f45bbd4 /compiler/utils | |
| parent | 37a6a52facd1c3999ce4472c50b0030568be1e04 (diff) | |
| parent | 3d56d5ae395675717d695172cacdd61c1565c42d (diff) | |
| download | haskell-cbebca1c9164a5e5ae9b117d0dcf5ad217defc6d.tar.gz | |
Merge ghc-new-co into master branch
Diffstat (limited to 'compiler/utils')
| -rw-r--r-- | compiler/utils/Pair.lhs | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/compiler/utils/Pair.lhs b/compiler/utils/Pair.lhs new file mode 100644 index 0000000000..eb594af45f --- /dev/null +++ b/compiler/utils/Pair.lhs @@ -0,0 +1,47 @@ +
+A simple homogeneous pair type with useful Functor, Applicative, and
+Traversable instances.
+
+\begin{code}
+module Pair ( Pair(..), unPair, toPair, swap ) where
+
+#include "HsVersions.h"
+
+import Outputable
+import Data.Monoid
+import Control.Applicative
+import Data.Foldable
+import Data.Traversable
+
+data Pair a = Pair { pFst :: a, pSnd :: a }
+-- Note that Pair is a *unary* type constructor
+-- whereas (,) is binary
+
+-- The important thing about Pair is that it has a *homogenous*
+-- Functor instance, so you can easily apply the same function
+-- to both components
+instance Functor Pair where
+ fmap f (Pair x y) = Pair (f x) (f y)
+
+instance Applicative Pair where
+ pure x = Pair x x
+ (Pair f g) <*> (Pair x y) = Pair (f x) (g y)
+
+instance Foldable Pair where
+ foldMap f (Pair x y) = f x `mappend` f y
+
+instance Traversable Pair where
+ traverse f (Pair x y) = Pair <$> f x <*> f y
+
+instance Outputable a => Outputable (Pair a) where
+ ppr (Pair a b) = ppr a <+> char '~' <+> ppr b
+
+unPair :: Pair a -> (a,a)
+unPair (Pair x y) = (x,y)
+
+toPair :: (a,a) -> Pair a
+toPair (x,y) = Pair x y
+
+swap :: Pair a -> Pair a
+swap (Pair x y) = Pair y x
+\end{code}
\ No newline at end of file |