I encountered a situation where using a tuple constraint synonym produced an asymptotic runtime performance degradation, from linear to quadratic time. The enclosed Bug.hs reproduces the potentially relevant features of the problem. The function test there is quadratic, but should be linear. The file also contains a definition of test2, which differs only in eschewing the constraint synonym, and is linear, as it should be.

In more detail: the program tries to count to 20,000 in a rather contrived way. It maintains a Box, which holds the current count and a function that can accept an increment and a count to produce a new count (namely, (+)). The function do_step lifts incrementation to Boxes, using their contained function to update their stored count.

The first tricky thing is that the incrementing function stored in the Box is polymorphic in both arguments separately: it can accept increments that are not the same type as the stored count (and converts them). The second tricky thing is that I don't want to expose the type variable for the increment type (as opposed to the count type) to clients of the Box, so the Box's incrementing function is stored polymorphic (with an explicit forall). Finally, I want to impose the constraint (Fractional num, Real num) on allowable increments (even though this example only needs Real num).

This constellation of desiderata conspires to produce quadratic performance: doubling the parameter to test (but not test2) multiplies its runtime by 4. Inspecting the heap profile indicates a growing accumulation of closures when running test (but not test2). Inspecting the generated stg (enclosed) locates a potential culprit: apparently when do_step (but not do_step2) reconstructs the Box, it changes the func stored there to be a fresh closure that destructures the Numerical constraint tuple, constructs a fresh one, and calls the function that was in that slot previously with it. I hypothesize that this accumulates as a chain that performs a linear number of such destructurings and restructurings at each increment, leading to the observed quadratic runtime and linear memory growth.

Incidentally, I checked whether record wildcards were the issue (no: test4) and whether updating just the obj field solves it (yes: test3). Sadly, the latter solution is not directly usable in my actual application because of "Record update for insufficiently polymorphic field".

For reference: I compiled with ghc -O2 Bug.hs -fprof-auto -rtsopts -ddump-to-file -ddump-simpl -ddump-st