For debugging my haskell code I first determine if the bug is in a pure function or IO. If it's in IO I use typical print type debugging (either running the IO action in ghci or just rerunning my program repeatedly).
If it's in a pure function, I load the code up in ghci and use it to replicate and explore the inputs that lead to the bug, and try out fixes. In a complex function I'll often promote (de-indent) any 'where/let' helpers to toplevel definitions so I can try them in ghci and narrow down which is behaving badly.
I have not missed gdb in haskell, but then most of my gdb experiences have involved asking for a stack trace and discovering the stack's been smashed by the bug. Or using gdb to chase down some unexpected mutation of a variable deep in the code. Not problems typical to haskell.
If it's in a pure function, I load the code up in ghci and use it to replicate and explore the inputs that lead to the bug, and try out fixes. In a complex function I'll often promote (de-indent) any 'where/let' helpers to toplevel definitions so I can try them in ghci and narrow down which is behaving badly.
I have not missed gdb in haskell, but then most of my gdb experiences have involved asking for a stack trace and discovering the stack's been smashed by the bug. Or using gdb to chase down some unexpected mutation of a variable deep in the code. Not problems typical to haskell.