The Weak Call-By-Value λ-Calculus is Reasonable for Both Time and Space

Saarland University Computer Science

Yannick Forster, Fabian Kunze, Marc Roth

We study the weak call-by-value λ-calculus as a model for computational complexity theory and establish the natural measures for time and space – the number of beta-reductions and the size of the largest term in a computation – as reasonable measures with respect to the invariance thesis of Slot and van Emde Boas [STOC 84]. More precisely, we show that, using those measures, Turing machines and the weak call-by-value λ-calculus can simulate each other within a polynomial overhead in time and a constant factor overhead in space for all computations that terminate in (encodings) of 'true' or 'false'. We consider this result as a solution to the long-standing open problem, explicitly posed by Accattoli [ENTCS 18], of whether the natural measures for time and space of the λ-calculus are reasonable, at least in case of weak call-by-value evaluation.

Our proof relies on a hybrid of two simulation strategies of reductions in the weak call-by-value λ-calculus by Turing machines, both of which are insufficient if taken alone. The first strategy is the most naive one in the sense that a reduction sequence is simulated precisely as given by the reduction rules; in particular, all substitutions are executed immediately. This simulation runs within a constant overhead in space, but the overhead in time might be exponential. The second strategy is heap-based and relies on structure sharing, similar to existing compilers of eager functional languages. This strategy only has a polynomial overhead in time, but the space consumption might require an additional factor of log n, which is essentially due to the size of the pointers required for this strategy. Our main contribution is the construction and verification of a space-aware interleaving of the two strategies, which is shown to yield both a constant overhead in space and a polynomial overhead in time.