event: http://www.meetup.com/NY-Haskell/events/119371832/

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.