Personal Site of Adam Gordon Bell - Software Engineer
This post is a work in progress and represents my getting to understand how laziness works in haskell and how it affects performance. Summary of Laziness:
Lazy is good when:
Strict is good otherwise.
The key insight for me has been understanding that lazy evaluation means that functions take in a pointer to a thunk as arguments and then return a thunk as a value. Only looking inside those thunks inputs (via pattern matching or IO) actually forces anything.
This level of indirection improves composability, you can generate than reduce in a more modular fashion with only some constant overhead for holding on to thunks. The paper “why functional programming matters” has a great explanation of this.
This all works because whenever you throw something away with i.e. f (a,) = a, the pointer to the thunk, that was never evaluated, and all the evaluations that follow from can get garbage collected away.
This how foldr can short-circuit on some lazy list, if a condition of f that is passed in doesn’t care about the rest of the list, then we are done.