Thunk Lifting: Reducing Heap Usage in an Implementation of a Lazy Functional Language

Thunk-lifting is a programtransformation for lazy functional programs. The transforma- tion aims at reducing the amount of heap space allocated to the program when it executes. Thunk-lifting transforms a function application that contains as arguments further, nested, function applications into a new function application without nesting. The transformation thus essentially folds some function applications. The applications to be folded are selected on the basis of a set of conditions, which have been chosen such that thunk-lifting never increases the amount of heap space required by a transformed program. Thunk-lifting has been implemented and applied to a number ofmediumsize benchmark programs. The results show that the number of cell claims in the heap decreases on average by 5%, with a maximum of 16%.