GHCi on Acid

GHCi on Acid is an extension to GHCi (Interactive GHC) for adding useful lambdabot features. This post details how to use it.

I use GHCi inside Emacs, so having GOA additionally.. increases my productivity.

What does it do?

Here are some examples of the commands that can be used.

The :instances command shows all the instances of a class:

GOA> :instances Monad
 ((->) r), ArrowMonad a, Cont r, ContT r m, Either e, ErrorT e m, IO,
 Maybe, RWS r w s, RWST r w s m, Reader r, ReaderT r m, ST s, State s,
 StateT s m, Writer w, WriterT w m, []
GOA> :instances Arrow
 (->), Kleisli m
GOA> :instances Num
 Double, Float, Int, Integer

Here we have the :hoogle command, for querying the Hoogle database. Great for looking for functions of a specific type:

GOA> :hoogle Arrow
 Control.Arrow :: module
Control.Arrow.Arrow :: class Arrow a
Control.Arrow.ArrowZero :: class Arrow a => ArrowZero a
GOA> :hoogle b -> (a -> b) -> Maybe a -> b
 Prelude.maybe :: b -> (a -> b) -> Maybe a -> b
Data.Maybe.maybe :: b -> (a -> b) -> Maybe a -> b

The :source command gives a link to the source code of a module (sometimes you are curious):

GOA> :source Data.Maybe Similarly, :docs gives a link to the documentation of a module.

GOA> :docs Data.Maybe

:index is a nice way to search modules.

GOA> :index Monad
 Control.Monad, Prelude, Control.Monad.Reader,
 Control.Monad.Writer, Control.Monad.State, Control.Monad.RWS,
 Control.Monad.Identity, Control.Monad.Cont, Control.Monad.Error,

Then we have :pl, which shows the pointless (or: point-free) way of writing a function, which is very useful for learning and sometimes for fun:

GOA> :pl (\x -> x * x)
 join (*)
GOA> :pl (\x y -> (x * 5) + (y * 5))
 (. (5 *)) . (+) . (5 *)

And finally, the best of them all; :bs:

GOA> :bs


How to install it

You need to download and install lamdabot. This is easier than it sounds.

Install happy

First, we need “happy”, which is a parser:

sudo cabal install happy

Right, that’s installed to ~/.cabal/bin, so you need to make sure ~/.cabal/bin is in your PATH. In Bash, you can do this by adding

export PATH="~/.cabal/bin:$PATH"

to your .bashrc.

Try it out:

chris@chrisdesktop:~$ happy --version
Happy Version 1.17 Copyright (c) 1993-1996 Andy Gill, Simon Marlow (c) 1997-2005 Simon Marlow

Happy is a Yacc for Haskell, and comes with ABSOLUTELY NO WARRANTY.
This program is free software; you can redistribute it and/or modify
it under the terms given in the file 'LICENSE' distributed with
the Happy sources.
If yours isn’t found, you need to restart bash, or type export PATH="~/.cabal/bin:$PATH".

Grab Hoogle

We need Hoogle for lambdabot’s Hoogle functionality.

sudo cabal install hoogle

Download and build lambdabot

You need darcs. We’re using darcs. It should be on your package manager, otherwise install it from the web site. I don’t know why it’s not on Hackage.

Now, we grab lambdabot from

darcs get

cd lambdabot && sudo ./build

./build downloads and installs everything we need, and builds lambdabot for us.

Once that’s built, we can install GOA.

Download, build and install GOA

Just install with your usual cabal install goa method. :)

Finally, copy the dot-ghci from goa/ to your ~/.ghci file, and edit the lambdabot home directory to where your lambdabot directory is. Mine looks like this:

:m - Prelude
:m + GOA
setLambdabotHome "/var/www/chrisdone/lambdabot-personal"
:def bs        lambdabot "botsnack"
:def pl        lambdabot "pl"
:def index     lambdabot "index"
:def docs      lambdabot "docs"
:def instances lambdabot "instances"
:def hoogle    lambdabot "hoogle"
:def source    lambdabot "fptools"
:def where     lambdabot "where"
:def version   lambdabot "version"

Now when you run ghci, you will have GOA! Tip: if you accidentally unload the GOA module, use :m + GOA to load it.