A set of variables that are in scope at some point "Secrets of the Glasgow Haskell Compiler inliner" Section 3. provides the motivation for this abstraction.

Look up a variable the InScopeSet. This lets you map from the variable's identity (unique) to its full value.

uniqAway in_scope v finds a unique that is not used in the in-scope set, and gives that to v.

When we are comparing (or matching) types or terms, we are faced with "going under" corresponding binders. E.g. when comparing:

\x. e1     ~   \y. e2

Basically we want to rename [x -> y] or [y -> x], but there are lots of things we must be careful of. In particular, x might be free in e2, or y in e1. So the idea is that we come up with a fresh binder that is free in neither, and rename x and y respectively. That means we must maintain:

1. A renaming for the left-hand expression
2. A renaming for the right-hand expressions
3. An in-scope set

Furthermore, when matching, we want to be able to have an 'occurs check', to prevent:

\x. f   ~   \y. y

matching with [f -> y]. So for each expression we want to know that set of locally-bound variables. That is precisely the domain of the mappings 1. and 2., but we must ensure that we always extend the mappings as we go in.

All of this information is bundled up in the RnEnv2

rnBndr2 env bL bR goes under a binder bL in the Left term, and binder bR in the Right term. It finds a new binder, new_b, and returns an environment mapping bL -> new_b and bR -> new_b

Applies rnBndr2 to several variables: the two variable lists must be of equal length

Similar to rnBndr2 but returns the new variable as well as the new environment

Look up the renaming of an occurrence in the left or right term

Look up the renaming of an occurrence in the left or right term

Tells whether a variable is locally bound

Tells whether a variable is locally bound

Look up the renaming of an occurrence in the left or right term

Look up the renaming of an occurrence in the left or right term

Similar to rnBndr2 but used when there's a binder on the left side only.

Similar to rnBndr2 but used when there's a binder on the right side only.

Wipe the left or right side renaming

Wipe the left or right side renaming

swap the meaning of left and right

Similar to rnBndrL but used for eta expansion See Note [Eta expansion]

Similar to rnBndr2 but used for eta expansion See Note [Eta expansion]

Retrieve the left mapping

Retrieve the right mapping

When tidying up print names, we keep a mapping of in-scope occ-names (the TidyOccEnv) and a Var-to-Var of the current renamings