We present the HOLCF-Prelude, a formalization of a large part of Haskell’s standard prelude in Isabelle/HOLCF. Applying this formalization to the hints suggested by HLint allows us to certify them formally. In pure functional languages such as Haskell, equational reasoning is a valuable tool for refactoring, to improve both efficiency and aesthetics. For example, an experienced programmer would...
