CODEWITHSUNDEEP
haskellghc
Doug McClean
Doug McClean

Reputation: 14485

What does the GHC source mean by "zonk"?

I'm working on a plugin for GHC, so I'm reading the documentation for some of its implementation.

The verb "to zonk" is all over the place, but I can't track down an explanation of what it means to zonk something or (in broad terms) when one might want to. I can find plenty of notes about complicated circumstances under which it is necessary to zonk or not to zonk something, but without a clue as to what the big picture is I am having a lot of trouble following.

Upvotes: 45

Views: 1967

Answers (1)

Cactus
Cactus

Reputation: 27636

An un-zonked type can have type variables which are mutable references filled in during unification (and this mutability is heavily used by the type checker to increase performance). Zonking traverses a type and replaces all mutable references with the type that they dereference to; thus, the resulting structure is immutable and requires no dereferencing to interpret.

Note that these type variables are meta-variables, i.e. they don't correspond to the type variables introduced by polymorphism; rather, they are unification variables to be replaced by real types. The choice of replacement is decided by the type checking/type inference process, and then the actual replacement is done during zonking.

This notion of zonking extends naturally to other intermediate representations of the typechecker that contain types.

Upvotes: 38

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