diff options
| -rw-r--r-- | libraries/base/Control/Arrow.hs | 11 |
1 files changed, 4 insertions, 7 deletions
diff --git a/libraries/base/Control/Arrow.hs b/libraries/base/Control/Arrow.hs index c9281569f9..1cc6062516 100644 --- a/libraries/base/Control/Arrow.hs +++ b/libraries/base/Control/Arrow.hs @@ -93,16 +93,14 @@ class Category a => Arrow a where -- | Send the first component of the input through the argument -- arrow, and copy the rest unchanged to the output. first :: a b c -> a (b,d) (c,d) + first = (*** id) -- | A mirror image of 'first'. -- -- The default definition may be overridden with a more efficient -- version if desired. second :: a b c -> a (d,b) (d,c) - second f = arr swap >>> first f >>> arr swap - where - swap :: (x,y) -> (y,x) - swap ~(x,y) = (y,x) + second = (id ***) -- | Split the input between the two argument arrows and combine -- their output. Note that this is in general not a functor. @@ -110,7 +108,8 @@ class Category a => Arrow a where -- The default definition may be overridden with a more efficient -- version if desired. (***) :: a b c -> a b' c' -> a (b,b') (c,c') - f *** g = first f >>> second g + f *** g = first f >>> arr swap >>> first g >>> arr swap + where swap ~(x,y) = (y,x) -- | Fanout: send the input to both argument arrows and combine -- their output. @@ -141,8 +140,6 @@ class Category a => Arrow a where instance Arrow (->) where arr f = f - first f = f *** id - second f = id *** f -- (f *** g) ~(x,y) = (f x, g y) -- sorry, although the above defn is fully H'98, nhc98 can't parse it. (***) f g ~(x,y) = (f x, g y) |