Debugging loop in haskell

Question

Say I had the following code in Haskell:

x :: Int
y :: Int
z :: Int
x=y
y=z
z=x

Naturally, referring to any of these variables causes execution to bomb with <<loop>> if using ghc or spin infinitely if using ghci. I'm working on some code that has high potential for circularity if miscoded - is there any way to debug <<loop>> - can I examine the stack when it bombs to see the circularity?

http://stackoverflow.com/questions/6724434/how-to-debug-haskell-code — The Internet, Oct 05 '15 at 02:42

score 2 · Accepted Answer · edited May 23 '17 at 12:15

I looked at the question @TheInternet linked, and the first answer's advice to use ~~-fbreak-on-exception~~-fbreak-on-error and :trace is pretty good. However I thought I'd mention two small snags I hit that are specific to this question:

GHCi does not make <<loop>> exceptions, only native compiled GHC does that, and the GHCi debugger cannot trace native compiled code.

Fortunately, this can be solved by pressing ^C to interrupt manually.

Trying to :trace and then ^C manually instead does give a debugger session, but GHCi hasn't inserted any breakpoints and so there's no context to debug!

This was also easy to fix: I just changed the first equation to x = id y. This is enough to get GHCi to insert breakpoints. I suspect that this problem was due to the OP's code cycle containing only trivial variable equations (to quote the GHC User's Guide: "Single variables are typically not considered to be breakpoint locations (unless the variable is the right-hand-side of a function definition, lambda, or case alternative)"), and that it will not usually happen in "real" code.

Debugging loop in haskell

1 Answers1