Monthly Archives: March 2018

The Task abstraction

Neil Mitchell, Simon Peyton Jones and I have just finished a paper describing a systematic and executable framework for developing and comparing build systems. The paper and associated code are available here: The code is not yet well documented and polished, but I’ll bring it in a good shape in April. You can learn more about the motivation behind the project here.

(Update: the paper got accepted to ICFP! Read the PDF, watch the talk.)

In this blog post I would like to share one interesting abstraction that we came up with to describe build tasks:

type Task c k v = forall f. c f => (k -> f v) -> k -> Maybe (f v)

A Task is completely isolated from the world of compilers, file systems, dependency graphs, caches, and all other complexities of real build systems. It just computes the value of a key k, in a side-effect-free way, using a callback of type k → f v to find the values of its dependencies. One simple example of a callback is Haskell’s readFile function: as one can see from its type FilePath → IO String, given a key (a file path k = FilePath) it can find its value (the file contents of type v = String) by performing arbitrary IO effects (hence, f = IO). We require task descriptions to be polymorhic in f, so that we can reuse them in different computational contexts f without rewriting from scratch.

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