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Garbage collector notification +g,-g

It is sometimes useful to monitor uses of the garbage collector, and to determine how many cells are recovered with each collection. If the +g option is set, then the interpreter will print a message of the form {{Gc:num}} each time that the garbage collector is invoked. The number after the colon indicates the total number of cells that are recovered.

As a simple application, we can use garbage collector messages to observe that an attempt to sum an infinite list, although non-terminating, will at least run in constant space: 

 Prelude> :set +g
 Prelude> sum [1..]
 {{Gc:95763}}{{Gc:95760}}{{Gc:95760}}{{Gc:95760}}{Interrupted!}

 Prelude>
Garbage collector messages may be printed at almost any stage in a computation (or indeed while loading, type checking or compiling a file of definitions). For this reason, it is often best to turn garbage collector messages off (using :set -g, for example) if they are not required.