With MLton and mlprof, you can profile your program to find out how many bytes each function allocates. To do so, compile your program with -profile alloc. For example, suppose that list-rev.sml is the following.
fun append (l1, l2) = case l1 of  => l2 | x :: l1 => x :: append (l1, l2) fun rev l = case l of  =>  | x :: l => append (rev l, [x]) val l = List.tabulate (1000, fn i => i) val _ = 1 + hd (rev l)
Compile and run list-rev as follows.
% mlton -profile alloc list-rev.sml % ./list-rev % mlprof -show-line true list-rev mlmon.out 6,030,136 bytes allocated (108,336 bytes by GC) function cur ----------------------- ----- append list-rev.sml: 1 97.6% <gc> 1.8% <main> 0.4% rev list-rev.sml: 6 0.2%
The data shows that most of the allocation is done by the append function defined on line 1 of list-rev.sml. The table also shows how special functions like gc and main are handled: they are printed with surrounding brackets. C functions are displayed similarly. In this example, the allocation done by the garbage collector is due to stack growth, which is usually the case.
The run-time performance impact of allocation profiling is noticeable, because it inserts additional C calls for object allocation.
Compile with -profile alloc -profile-branch true to find out how much allocation is done in each branch of a function; see ProfilingCounts for more details on -profile-branch.