From 5846864d6321bb3c686ba1c9c16c132fbc171b5e Mon Sep 17 00:00:00 2001
From: Edward Kmett <ekmett@gmail.com>
Date: Mon, 9 Feb 2015 22:35:17 -0500
Subject: [PATCH] Added `like` and `ilike`.

---
 src/Control/Lens/Getter.hs | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/Control/Lens/Getter.hs b/src/Control/Lens/Getter.hs
index 7780e64d0..8389864d8 100644
--- a/src/Control/Lens/Getter.hs
+++ b/src/Control/Lens/Getter.hs
@@ -51,6 +51,8 @@ module Control.Lens.Getter
   -- * Building Getters
   , to
   , ito
+  , like
+  , ilike
   -- * Combinators for Getters and Folds
   , (^.)
   , view, views
@@ -131,6 +133,33 @@ ito :: (Indexable i p, Contravariant f) => (s -> (i, a)) -> Optical' p (->) f s
 ito k = dimap k (contramap (snd . k)) . uncurry . indexed
 {-# INLINE ito #-}
 
+
+-- | Build an constant-valued (index-preserving) 'Getter' from an arbitrary Haskell value.
+--
+-- @
+-- 'like' a '.' 'like' b ≡ 'like' b
+-- a '^.' 'like' b ≡ b
+-- a '^.' 'like' b ≡ a '^.' 'to' ('const' b)
+-- @
+--
+-- This can be useful as a second case 'failing' a 'Fold'
+-- e.g. @foo `failing` 'like' 0@
+--
+-- @
+-- 'like' :: a -> 'IndexPreservingGetter' s a
+-- @
+like :: (Profunctor p, Contravariant f) => a -> Optical' p p f s a
+like a = to (const a)
+{-# INLINE like #-}
+
+-- |
+-- @
+-- 'ilike' :: i -> a -> 'IndexedGetter' i s a
+-- @
+ilike :: (Indexable i p, Contravariant f) => i -> a -> Optical' p (->) f s a
+ilike i a = ito (const (i, a))
+{-# INLINE ilike #-}
+
 -- | When you see this in a type signature it indicates that you can
 -- pass the function a 'Lens', 'Getter',
 -- 'Control.Lens.Traversal.Traversal', 'Control.Lens.Fold.Fold',