# Using Hoogle to Search for Haskell Functions

# Using Hoogle to Search for Haskell Functions

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

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## 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.

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

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