stepping through a function line by line

Question

This user guide:

http://www.haskell.org/ghc/docs/latest/html/users_guide/ghci-debugger.html

advertises:

Execution can be single-stepped: the evaluator will suspend execution approximately after every reduction, allowing local variables to be inspected. This is equivalent to setting a breakpoint at every point in the program.

Yet, I can find nothing in the document that tells me how to do that. Under the heading:

2.5.2. Single-stepping

It describes how to step from breakpoint to breakpoint. But I don't want to have to set a breakpoint on every line. Was the advertisement false or is there a way to step through a program line by line?

Thanks.

Answer 1

After having set and reached a breakpoint, you can call :step from the debugger.

There are other single-step possibilities. Typing :help once at a breakpoint would tell you more about what you can do.

Answer 2

Okay, I figured it out:

ghci> :step function_name arg1 arg2
...
...
ghci> :step   
...
...
ghci> :step

If you forget the function arguments, then you will get the cryptic error message:

<interactive>:138:1:
    No instance for (Show (String -> Double))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for (Show (String -> Double))
    In a stmt of an interactive GHCi command: print it

...which might lead you to tear your hair out. And if you want to skip to the end:

ghci> :continue