[chadaustin]

For values, laziness means there is a tag bit for whether a value is a thunk or evaluated. Sum types use tags to determine which variant is active.

For functions, because a function that takes two arguments and returns a value (a -> a -> a) has the same type as a function that takes one argument and returns a function that takes another argument that returns a value (a -> a -> a), the arity of functions is stored in the tag.

Some of these tags are eliminated by inlining but if you sit down and read some typical Haskell output you'll see a _whole lot_ of tag checks.

Source: spent a lot of time reading GHC output and writing high-performance Haskell code.