What is hoogle?
Hoogle is Google for searching of Haskell functions. You could ask it for function name or its signature. There is available command hoogle, which could be installed using stack:
$ stack install hoogle
Using hoogle from command line {#using-hoogle-from-command-line} To
hoogle a function you could just pass it as parameter:
$ hoogle fmap
Prelude fmap :: Functor f => (a -> b) -> f a -> f b
Data.Functor fmap :: Functor f => (a -> b) -> f a -> f b
Control.Monad fmap :: Functor f => (a -> b) -> f a -> f b
Control.Monad.Instances fmap :: Functor f => (a -> b) -> f a -> f b
Data.Traversable fmapDefault :: Traversable t => (a -> b) -> t a -> t b
Network.Stream fmapE :: (a -> Result b) -> IO (Result a) -> IO (Result b) or pass its signature:
$ hoogle "(a -> b -> c) -> [a] -> [b] -> [c]"
Prelude zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
Data.List zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
Control.Applicative liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
Control.Monad liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
Prelude scanl :: (a -> b -> a) -> a -> [b] -> [a]
Data.List scanl :: (a -> b -> a) -> a -> [b] -> [a]
Prelude scanr :: (a -> b -> b) -> b -> [a] -> [b]
Data.List scanr :: (a -> b -> b) -> b -> [a] -> [b]
Data.List deleteFirstsBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
Data.List intersectBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
... But it shows only list of the signatures of the functions. Sometimes we want to see more information about function. If you use option
-i, then additional information will be shown:
$ hoogle -i “(a -> b -> c) -> [a] -> [b] -> [c]”
Prelude zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWith generalises zip by zipping with the function given as the first argument, instead of a tupling function. For example, zipWith (+) is applied to two lists to produce the list of corresponding sums.
From package base
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] And again it is not enough, because it shows only documentation of first function from the list. We have to move a counter to see documentation of further functions, e. g. to show information about third item from the list:
$ hoogle -i -s 3 "(a -> b -> c) -> [a] -> [b] -> [c]"
Control.Applicative liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
Lift a binary function to actions.
From package base
liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
Easier hoogle usage in bash {#easier-hoogle-usage-in-bash} It is cumbersome to count each time you want to read info about further functions, so I have prepared bash function which makes it easier. To search for function type:
$ hoog "(a->b) -> f a -> f b"
1) Data.Traversable fmapDefault :: Traversable t => (a -> b) -> t a -> t b
2) Prelude fmap :: Functor f => (a -> b) -> f a -> f b
3) Data.Functor fmap :: Functor f => (a -> b) -> f a -> f b
4) Control.Monad fmap :: Functor f => (a -> b) -> f a -> f b
5) Control.Monad.Instances fmap :: Functor f => (a -> b) -> f a -> f b
6) Data.Functor (<$>) :: Functor f => (a -> b) -> f a -> f b
7) Control.Applicative (<$>) :: Functor f => (a -> b) -> f a -> f b Each function will have its counter at the beginning and just add its number at the end of command to show more information about specific function:
$ hoog "(a->b) -> f a -> f b" 6
Searching for: (a -> b) -> f a -> f b
Data.Functor (<$>) :: Functor f => (a -> b) -> f a -> f b
An infix synonym for fmap.
From package base
(<$>) :: Functor f => (a -> b) -> f a -> f b
How to install hoog command? {#how-to-install-hoog-command} Command is available
here. To use this command just add it to your ~/bashrc file.