StepULC

Efficient and single-steppable ULC (untyped lambda calculus) evaluation algorithm

The algorithm mainly comes from the following PEPM '17 paper:

Berezun, Daniil & Jones, Neil. (2017). Compiling untyped lambda calculus to lower-level code by game semantics and partial evaluation (invited paper). 1-11. 10.1145/3018882.3020004.

Specifically, Semantics 1 through 3 from Chapter 3 of the paper are implemented with certain modifications, in Haskell.

The main objectives of this development are:

• Directly work with de Bruijn indexes from the start.
• Avoid cloning and modifying substructures of a ULC expression (semi-compositionality).
• Allow single-stepping at the algorithm level (tail recursion).
• Evaluate the normal form, if it exists.
• Avoid any tricks related to lazy evaluation, so it can be easily ported to other languages (especially imperative ones).

It was highly non-trivial to modify the presented algorithms to the ones that actually evaluate the normal form, but I finally did it by following the incremental transformations as presented in the paper. And the last one, semantic3, avoids all lazy shenanigans like infinite lists and composed functions inside a data structure, so that it can be ported to more imperative languages.

Regarding performance, a (somewhat suboptimal) factorial function was used (fix is the fixpoint combinator, and other named functions are known ones for Church numerals):

fac = fix facFix where
    facFix = \f n. (iszero n) (\f x. f x) (mult n (f (pred n)))

Naive repeated beta reduction takes multiple hundred milliseconds to evaluate fac 4. semantic2 avoids cloning substructures, and takes under 10 milliseconds for the same input. (Exact timings vary wildly, but I observed around x50 ~ x100 speedup.) Further modifications do not improve in terms of execution time in Haskell (semantic3 is 2~4 times slower than semantic2), but they are essential in recovering the "single-steppability" and portability.