Over a million developers have joined DZone.

Using Hoogle to Search for Haskell Functions

An easy way to use Hoogle from bash to search for functions in Haskell.

· Big Data Zone

Learn how you can maximize big data in the cloud with Apache Hadoop. Download this eBook now. Brought to you in partnership with Hortonworks.

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

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 a 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

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?

Command is available here. To use this command just add it to your ~/bashrc file.

Hortonworks DataFlow is an integrated platform that makes data ingestion fast, easy, and secure. Download the white paper now.  Brought to you in partnership with Hortonworks

Topics:
haskell ,functional programming ,functional languages ,documentation

Published at DZone with permission of Dominik Przybysz. See the original article here.

Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

SEE AN EXAMPLE
Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.
Subscribe

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}