This document discusses using GHCi, Haskell's interactive environment, more effectively. It provides tips on allowing multi-line definitions, showing types, executing Haskell code, and setting up GHCi. It also covers loading and reloading modules, compiled vs interpreted code, controlling GHCi scope, debugging basics like breakpoints, and advanced techniques like time travel debugging.

1. -- GHCi --
More Awesome
Than You (I?) Thought
Richard Minerich
Senior Researcher at Bayard Rock
@Rickasaurus

2. Noob Answers for
Noob Questions
How do I allow multi-line definitions?
Wrap it in “:{ … :}” or “:set +m”
Why doesn’t it show the type of my result?
“:set +t” will enable type info.
Why won’t my Haskell code execute in GHCi?
GHCi uses the monad “let” syntax.
How do I specify the type of something?
Follow it with “:: type” (e.g x = 5 :: Int)

3. Setup for this talk
Sublime Text 2 with
- SublimeHaskell
- SublimeREPL (head)
- Tabs to Spaces on
Haskell Platform (2012.4.0.0) with
aeson, haskell-src-exts, haddock,
ghc-mod, stylish-haskell, haskell-docs

4. SublimeREPL
SublimeREPL (as of last week) will now:
- Intelligently remove extra whitespace in
front of nested functions
- Prepends “let “ to functions sent to the REPL
and indents the remainder (as needed)
- Automatically wraps multi-line functions with
the new block syntax :{ .. :}

5. Haskell for Scripting
-- script.hs --
#!/usr/bin/env runhaskell
main = putStrLn “Hello, or something…”
On Windows:
runhaskell script.hs
On Posix:
./script.hs

6. GHCi Custom Configs
Search Paths:
:set +t
:def hoogle str ->
return $ ":! hoogle --count=15 "" ++ str ++ """
Tons of useful macros at the wiki:
http://www.haskell.org/haskellwiki/GHC/GHCi
./.ghci $HOME/.ghc/ghci.conf
appdata/ghc/ghci.conf $HOME/.ghci

7. Play with Language Flags
{-# LANGUAGE NoMonomorphismRestriction #-}
Vs.
:set –XNoMonomorphismRestriction
Vs.
(in latest Haskell Platform)
:seti –XNoMonomorphismRestriction

8. Loading Modules
By default module Dir.Module is Dir/Module.hs
:load Dir.Module Load compiled if exists
:load *Dir.Module Load interpreted forced
:reload Reloads target set
:show modules List loaded modules

9. Compiled vs. Interpreted
Compiled Interpreted
~10x Faster Execution ~2x Faster Load
Dependencies must be
compiled
Dependencies compiled
or interpreted
Only exports available Exports or full scope
“:! ghc –c Module.hs” Noop

10. Controlling GHCi Scope
:show imports
Shows what’s in scope
:module – Dir1.Module1 …
Remove “./Dir1/Module1.hs” from scope
:module + Dir1.Module1 *Dir2.Module2 …
Open exports from “./Dir1/Module1.hs”
Open full scope from “./Dir2/Module2.hs”
Standard import syntax applies as well

11. Your Eyes and Ears
:browse Module - Shows a module’s exports
:browse *Module – Shows full Module scope
:show bindings – List bound variables with type
:type <expr> - Type information for an expr
:info <name> - Extended information
:print <id> - Print without forced evaluation
:force <id> - Print with forced evaluation

12. GHCi Funny Business
GHCi is a monad, monad syntax applies (kind of)
Bindings go away after “:load” or “:reload”
Type defaults (are strange)
It’s thunks all the way down
“print” vs “:print” vs “:sprint” vs “:force”

13. Things to know about Debugging
All the standard stuff:
- Breakpoints, Break on Exception, Stepping
And some fancy stuff:
- Time Travel, Subexpression Breakpoints
But, some oddities (coming from other langs):
- Scope, Execution Ordering, :Print Breakpoints

14. Review: Debugging Basics
:set stop :list
:break [<id>] or [<module?> <line?> <col?>]
:show breaks
:delete <breakpoint_id|*>
:step <expr?> / :steplocal / :stepmodule
:abandon / :continue
:print <id> / :force <id>

15. Review: Time Travel Wizardry
:trace <expr>
:hist <num?>
:back / :forward
:show context
:set –fbreak-on-exception
:set –fbreak-on-error
_result / _exception

16. How the heck do I trace?
Remember, Haskell is lazy. So be thoughtful:
- Break inside, near the end
- Leverage assertions with error
(import Control.Exception (assert))
- Give BangPatterns a go
- Debug from a secondary .hs file
- Implement conditional breakpoints

17. That was just the tip of the iceburg, see the
Docs and Wiki for MUCH more information.