CODEWITHSUNDEEP
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Robert Jakubowicz
Robert Jakubowicz

Reputation: 338

GHCi - Breakpoint is skipped on second run

I'm new to Haskell and I'm getting an annoying behaviour when debugging.

  1. I add my break point using :break
  2. I run main
  3. Everything is ok
  4. I type :continue to finish the execution

When I rerun main, the breakpoint does not hit anymore but the breakpoint wasn't removed because :show breaks lists it. Anyone knows what's going on?

I'm on Ubuntu 11.10, 64 bits. I'll test it on a different environment tomorrow.

Thanks

Upvotes: 3

Views: 207

Answers (2)

Maarten Faddegon
Maarten Faddegon

Reputation: 785

To avoid recomputations constant application forms are replaced with an indirection to its redex.

For example the following snippet the right hand side of 'papperlap' will be replaced with an indirection node pointing to '4'.

bla x = x + 1
papperlap = bla 3

If you set a breakpoint on 'bla' and ask for 'papperlap' twice you will see that 'bla' is applied only once. But if you ask for 'bla 3' twice, we will also stop twice:

*Main> :break bla
Breakpoint 0 activated at meerbla.hs:1:1-13
*Main> papperlap
Stopped at meerbla.hs:1:1-13
_result :: a = _
[meerbla.hs:1:1-13] *Main> :continue
4
*Main> papperlap
4
*Main> bla 3
Stopped at meerbla.hs:1:1-13
_result :: a = _
[meerbla.hs:1:1-13] *Main> :continue
4
*Main> bla 3
Stopped at meerbla.hs:1:1-13
_result :: a = _
[meerbla.hs:1:1-13] *Main> :continue
4

Upvotes: 0

danr
danr

Reputation: 2425

It's hard to know without seeing the code, but it sounds likely that on the second run of main the breakpoint is never reached because the result is cached because of lazy evaluation. It probably was a THUNK (a suspended evaluation) first time, and the second time it is already evaluated.

Upvotes: 2

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