Summary
In this chapter, we have covered the basic evaluation mechanism of functional programs, beta reduction, and evaluation strategies that decide in which order reductions are performed. The common Call by Value strategy may perform unnecessary reductions. This is prevented by Call by Name but at the cost of sometimes duplicating work. Haskell’s Call by Need (or lazy evaluation) mechanism combines the best of both: it only performs necessary reductions and never duplicates work. We can exploit this strategy for the purpose of streaming, often aided by infinite datatypes and corecursive functions. At the same time, we need to be careful about the memory allocations that arise from deferred reductions.
The next chapter explains how Haskell programs interface with their environment: the user, the file system, the operating system, and any other party outside of the program. This is particularly challenging due to lazy evaluation. Yet, Haskell has found a unique solution...