Higher-order type-level programming in Haskell

Type family applications in Haskell must be fully saturated. This means that all type-level functions have to be first-order, leading to code that is both messy and longwinded. In this paper we detail an extension to GHC that removes this restriction. We augment Haskell’s existing type arrow, |->...

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Bibliographic Details
Published in:	Proceedings of ACM on programming languages Vol. 3; no. ICFP; pp. 1 - 26
Main Authors:	Kiss, Csongor, Field, Tony, Eisenbach, Susan, Peyton Jones, Simon
Format:	Journal Article
Language:	English
Published:	01.08.2019
ISSN:	2475-1421, 2475-1421
Online Access:	Get full text
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Summary:	Type family applications in Haskell must be fully saturated. This means that all type-level functions have to be first-order, leading to code that is both messy and longwinded. In this paper we detail an extension to GHC that removes this restriction. We augment Haskell’s existing type arrow, \|->\|, with an unmatchable arrow, \| >\|, that supports partial application of type families without compromising soundness. A soundness proof is provided. We show how the techniques described can lead to substantial code-size reduction (circa 80%) in the type-level logic of commonly-used type-level libraries whilst simultaneously improving code quality and readability.
ISSN:	2475-1421 2475-1421
DOI:	10.1145/3341706