About

LocH is a small Haskell module providing source location-specific error messages and debugging strings for Haskell code. It uses the compiler-expanded 'assert' token, rather than cpp or m4, to provide a lightweight approach to generating source locations. No preprocessor is required.

Example

import Control.Monad.State
import Debug.Trace.Location

main =
    trace assert "Starting runState" $ do
    runStateT code [1..] >> return ()
    trace assert "Finished" $ do
    let x = 1 + 2
    failure assert "All dead now."

code = do
    x <- pop
    io $ print x
    io $ checkIO assert (readFile "/tmp/f")
    y <- pop
    io $ print y
    return ()

pop = do
    check assert (head [])
    (x:xs) <- get
    put xs
    return x

io = liftIO

Produces:

    $ ./a.out 
    A.hs:5:10-15: Starting runState
    a.out: A.hs:20:10-15: Prelude.head: empty list
    

Indicating that the use of 'head' on line 20 failed.

Locating errors mechanically

LocH also comes with a preprocessor, that can be used to insert located errors automatically. Given a program failing with 'fromJust: Nothing':

    $ ghc A.hs --make -no-recomp
    [1 of 1] Compiling Main             ( A.hs, A.o )
    Linking A ...
    
    $ ./A
    A: Maybe.fromJust: Nothing
    

To locate the error all we have to do is "import Debug.Trace.Location", and then recompile the source with the 'loch' preprocessor:

    $ ghc A.hs --make -pgmF loch -F -no-recomp
    [1 of 1] Compiling Main             ( A.hs, A.o )
    Linking A ...

    $ ./A
    A: A.hs:14:14-19: Maybe.fromJust: Nothing
    

Documentation

• Documentation

Get LocH

• LocH 0.2
• darcs get --partial http://www.cse.unsw.edu.au/~dons/code/loch

Project Activity