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04/09/07 11:37:51 (
=== Stopping at a breakpoint at runtime in the byte code interpreter ===
Unfortunately this story is somewhat complicated, ''c'est la vie''.
To understand what happens it is necessary to know how GHCi evaluates an expression at the command line. When the user types in an expression (as a string) it is parsed, type checked, and compiled, and then run. In `main/GHC.hs` we have the function:
runStmt :: Session -> String -> IO RunResult
The `Session` argument contains the piles of environmental information which is important to the compiler. The `String` is what the user typed in, and `RunResult` is the answer that you get back if the execution terminates.
Normally what happens is that `runStmt` forks a new thread to handle the evaluation of the expression. It then blocks on an `MVar` and waits for the thread to finish. When the thread finishes it fills in the `MVar`, which wakes up `runStmt`, and it returns a `RunResult`. Ultimately this gets passed back to the GHCi command line. Actually, GHCi is merely a ''client'' of the API, and other clients could also call `runStmt` if they wanted something evaluated.
To make the discussion comprehensible let us distinguish two threads:
1. The thread which runs the GHCi prompt.
2. The thread which is forked to run an expression.
We'll call the first one the ''GHCi thread'', and the second the ''expression thread''.
In the debugger the process of evaluating an expression is made more intricate. The reason is that if the expression thread hits a breakpoint it will want to return ''early'' to the GHCi thread, so that the user can access the GHCi prompt, issue commands ''etcetera''.