Weekly PL reading group discussing the paper "Let Arguments Go First". Discussion will be led by Yiyun Liu. Paper available at https://link.springer.com/chapter/10.1007/978-3-319-89884-1_10

Bi-directional type checking has proved to be an extremely useful and versatile tool for type checking and type inference. The conventional presentation of bi-directional type checking consists of two modes: inference mode and checked mode. In traditional bi-directional type-checking, type annotations are used to guide (via the checked mode) the type inference/checking procedure to determine the type of an expression, and type information flows from functions to arguments.

This paper presents a variant of bi-directional type checking where the type information flows from arguments to functions. This variant retains the inference mode, but adds a so-called application mode. Such design can remove annotations that basic bi-directional type checking cannot, and is useful when type information from arguments is required to type-check the functions being applied. We present two applications and develop the meta-theory (mostly verified in Coq) of the application mode.