Instead of ":trace" and ":trace <expr>" command there should be one command ":set trace on/off" and a new command ":debug <expr>".

":set trace" would control a global flag indicating whether tracing should be active or not. If tracing is active then:[[BR]]

• ":continue" would behave like current ":trace",[[BR]]
• forcing a value using ":force <expr>" woudl work like current ":trace <expr>" but ignoring breakpoitns,[[BR]]
• ":debug <expr>" would work like current ":trace <expr>",[[BR]]
• and ":main ..." would start Main.main with tracing on from the very beginning.[[BR]]

If tracing is not active then ":continue", ":force", ":main" would behave like they do now and ":debug <expr>" would be the same as current "<expr>".

Reasoning:

I believe people use tracing to get access to variables which are not free in the selected scope but which contributed to values in the selected scope. So if they want variable availability they want tracing on all the time if they care about speed they want tracing off all the time. With this change request accepted they do not need to remember which command to use to continue or to print a forced value. When an <expr> is an argument to a ghci command and tracing is on then trace hisotry is extended, if tracing is off then trace history is not extended, if <expr> is typed on the ghci command line directly then tracing is never extended.

This change is not that important for manual ussage but it helps to simplify custom defined ghci comamnds/scripts (which e.g. will not need to take care whether to use ":continue" or ":trace" based on a global flag). This would also allow to start tracing from the very beggining of ":main" instead of setting a break at Main.main with a script set to ":trace" and then running ":main ...".

Related discussion is here: http://www.haskell.org/pipermail/glasgow-haskell-users/2009-January/016436.html

I do not know about other usage patterns for which the current state of tracing UI is better. If you do know then vote against this.