GHCi debugger: new flag -fbreak-on-exception
When -fbreak-on-exception is set, an exception will cause GHCi to suspend the current computation and return to the prompt, where the history of the current evaluation can be inspected (if we are in :trace). This isn't on by default, because the behaviour could be confusing: for example, ^C will cause a breakpoint. It can be very useful for finding the cause of a "head " or a "fromJust Nothing", though.
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