Lazy evaluation sometimes makes it trickier to really understand how a piece of code for folks used to languages with strict semantics (as I am). Sometimes introducing strictness is necessary to avoid space leaks and to make memory allocations more predictable in certain parts of our code. The usual suggestion is to “carefully sprinkle strict evaluation” in our code; one of the classic examples of memory leak is using foldl to sum a list of ints, with the result that instead of returning a result using constant space, it ends up taking an outrageous amount of memory before returning a result because thunks pile up (this behaviour is known as space leak). Most of the times I personally find it tricky to add strictness to a piece of Haskell code, so I’d like to share my latest experience doing that.
We’ll be using the Bloom filter implemented in chapter 26 of Real World Haskell as an example, the version contained in the book creates the filter lazily: our goal will be to create a strict version of that particular piece of code.