Question

I have a problem. I wrote a big Haskell program, and it always works with small input. Now, when I want to test it and generate a bigger input, I always get the message:

HsProg: Prelude.head: empty list

I use Prelude.head many times. What can I do to find out more or get a better error output to get the code line in which it happens?

Answer 1

The GHCi option -fbreak-on-exception can be useful. Here’s an example debugging session. First we load our file into GHCi.

$ ghci Broken.hs
GHCi, version 7.0.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
[1 of 1] Compiling Main             ( Broken.hs, interpreted )
Ok, modules loaded: Main.

Now, we turn on -fbreak-on-exceptions and trace our expression (main in this case for the whole program).

*Main> :set -fbreak-on-exception
*Main> :trace main
Stopped at <exception thrown>
_exception :: e = _

We’ve stopped at an exception. Let’s try to look at the code with :list.

[<exception thrown>] *Main> :list
Unable to list source for <exception thrown>
Try :back then :list

Because the exception happened in Prelude.head, we can’t look at the source directly. But as GHCi informs us, we can go :back and try to list what happened before in the trace.

[<exception thrown>] *Main> :back
Logged breakpoint at Broken.hs:2:23-42
_result :: [Integer]
[-1: Broken.hs:2:23-42] *Main> :list
1  
2  main = print $ head $ filter odd [2, 4, 6]
3  

In the terminal, the offending expression filter odd [2, 4, 6] is highlighted in bold font. So this is the expression that evaluated to the empty list in this case.

For more information on how to use the GHCi debugger, see the GHC User’s Guide.