GHCi fails to garbage collect declaration `l = length [1..10^8]` entered at prompt
This was recently reported on Stack Overflow (https://stackoverflow.com/questions/48723119/binding-len-length-xs-and-then-calculating-len-causes-ghc-to-consume-lots).
With GHC 8.2.2, assigning the length of a large list as a top-level declaration at the GHCi prompt and evaluating it:
Prelude> l = length [1..10^8] Prelude> l 10000000 Prelude>
allocates about 7 gigs of memory before displaying a result, and then memory continues to grow over the next few seconds (at the idle GHCi prompt) until 10 gigs of memory has been allocated. None of it is freed until the interpreter session ends.
In contrast, directly evaluating the expression or assigning it with a let-statement does **not** exhibit this behavior:
Prelude> length [1..10^8] 10000000 Prelude> let l = length [1..10^8] Prelude> l 10000000 Prelude>
Also, loading the top-level declaration from a file and then evaluating it at the prompt does **not** exhibit the problem.
I took a look at Ticket 14336 and applied the
-fno-it patch referenced there. This slightly changed the pattern of memory usage: it grew to about 4 gigs before displaying the result, but then continued to grow for several seconds at the GHCi prompt before landing at about 10 gigs.
I verified that the bug still exists on the current "master" branch (though I didn't check the "ghc-8.4" branch).