A promise checked is a promise kept: inspection testing

Abstract

Occasionally, developers need to ensure that the compiler treats their code in a specific way that is only visible by inspecting intermediate or final compilation artifacts. This is particularly common with carefully crafted compositional libraries, where certain usage patterns are expected to trigger an intricate sequence of compiler optimizations -stream fusion is a well-known example.The developer of such a library has to manually inspect build artifacts and check for the expected properties. Because this is too tedious to do often, it will likely go unnoticed if the property is broken by a change to the library code, its dependencies or the compiler. The lack of automation has led to released versions of such libraries breaking their documented promises.This indicates that there is an unrecognized need for a new testing paradigm, inspection testing, where the programmer declaratively describes non-functional properties of an compilation artifact and the compiler checks these properties. We define inspection testing abstractly, implement it in the context of the Haskell Compiler GHC and show that it increases the quality of such libraries.

Theorem proving for all: equational reasoning in liquid Haskell (functional pearl)

“…One might worry that all the calls to reverse , reverseApp , (++) and assocP in the derivation will spoil the performance of the function, but because Haskell is a lazy language, in practice none of these calls are actually executed. And in fact (with optimizations turned on), the compiler completely removes them from the code and -as we confirmed using inspection testing [Breitner 2018] -both definitions of reverseApp optimize to identical intermediate code.…”

Faster coroutine pipelines: A reconstruction

“…Adapting these rules correctly required a few extra steps, which we discuss in the next two paragraphs. During development, the inspection-testing library by Breitner (2018) was a helpful tool to quickly test whether fusion was happening correctly.…”