Bug introduced in 10.0.0 and fixed in 10.0.2
Consider the following code.
Clear[f, g] f[x_, list_] := Function[i, If[MemberQ[list, {i, i}], N[x], 0]]; g[x_, list_] := Function[i, N[x]]; v = f[2, {{1, 1}}]; w = g[2, {{1, 1}}]; Then, running the code below reveals that v consumes much more memory than w, despite their similar definitions.
MemoryInUse[] v[1] & /@ Range[100000]; MemoryInUse[] w[1] & /@ Range[100000]; MemoryInUse[] Why is this so?
This appears to have been a bug that was present in version 10.0, but has been fixed in version 10.1.
I can reproduce the behaviour using Mathematica version 10.0.1, but the leak does not occur in versions 7.0.1, 8.0.1, 9.0.1, or 10.1 (all on Win7 64-bit). I initially set $HistoryLength = 0 to remove saved results from consideration.
I speculate that MemberQ in v10.0 is creating an intermediate structure for searching, and that structure is not being released after use. If I change the code to use !FreeQ instead, then the leak persists. The leak disappears if I define my own simplistic memberQ function:
memberQ[list_, v_] := Scan[If[#===v, Return[True]]&, list] /. Null -> False Whatever structure MemberQ is building, it is not reclaimed when I call ClearSystemCache.
MemberQ and FreeQ, introduced in 10.0.0 and fixed in 10.0.2 (and also 10.1.0). $\endgroup$