(Video) Being lazy without being bloated
Posted on September 11, 2020This presentation was part of the Munihac 2020.
Laziness is one of Haskell’s most distinctive features. It is one of the two features of functional programming that “Why Functional Programming Matters” identifies as key to modularity, but it is also one of the most frequently cited features of Haskell that programmers would perhaps like to change. One reason for this ambivalence is that laziness can give rise to space leaks, which can sometimes be fiendishly difficult to debug. In this talk we will present a new library called nothunks which can be used to test for the absence of unexpected thunks in long-lived data; when an unexpected thunk is found, a “stack trace” is returned identifying precisely where the thunk is (“the second coordinate of a pair in a map in a list in type T”). In combination with QuickCheck, this can be used to test that an API does not create any thunks when it shouldn’t and thunks that are created are easily identified and fixed. Whilst it doesn’t of course fix all space leaks, it can help avoid a significant proportion of space leaks due to excessive laziness.