Expanding the shorthand, the definition of toy-store@ could almost be written as
|(define inventory null)|
|(define (store-color) 'green)|
A difference between this expansion and define-unit is that the imports and exports of toy-store@ cannot be inferred. That is, besides combining define and unit, define-unit attaches static information to the defined identifier so that its signature information is available statically to define-values/invoke-unit/infer and other forms.
Despite the drawback of losing static signature information, unit can be useful in combination with other forms that work with first-class values. For example, we could wrap a unit that creates a toy store in a lambda to supply the store’s color:
|; In "toy-store-maker.ss":|
|(define inventory null)|
|(define (store-color) the-color)|
|; the rest is the same as before|
|(define (maybe-repaint t)|
|(if (eq? (toy-color t) (store-color))|
|(repaint t (store-color))))|
|(define (stock! n)|
|(define (get-inventory) inventory))))|
To invoke a unit created by toy-store@-maker, we must use define-values/invoke-unit, instead of the /infer variant:
|> (require "simple-factory-unit.ss")|
|> (define-values/invoke-unit/infer simple-factory@)|
|> (require "toy-store-maker.ss")|
|> (stock! 2)|
(#(struct:toy purple) #(struct:toy purple))
In the define-values/invoke-unit form, the (import toy-factory^) line takes bindings from the current context that match the names in toy-factory^ (the ones that we created by invoking simple-factory@), and it supplies them as imports to toy-store@. The (export toy-store^) clause indicates that the unit produced by toy-store@-maker will export toy-store^, and the names from that signature are defined after invoking the unit.
To link a unit from toy-store@-maker, we can use the compound-unit form:
|> (require "store-specific-factory-unit.ss")|
This compound-unit form packs a lot of information into one place. The left-hand-side TF and TS in the link clause are binding identifiers. The identifier TF is essentially bound to the elements of toy-factory^ as implemented by store-specific-factory@. The identifier TS is similarly bound to the elements of toy-store^ as implemented by toy-store@. Meanwhile, the elements bound to TS are supplied as imports for store-specific-factory@, since TS follows store-specific-factory@. The elements bound to TF are similarly supplied to toy-store@. Finally, (export TF TS) indicates that the elements bound to TF and TS are exported from the compound unit.
The above compound-unit form uses store-specific-factory@ as a first-class unit, even though its information could be inferred. Every unit can be used as a first-class unit, in addition to its use in inference contexts. Also, various forms let a programmer bridge the gap between inferred and first-class worlds. For example, define-unit-binding binds a new identifier to the unit produced by an arbitrary expression; it statically associates signature information to the identifier, and it dynamically checks the signatures against the first-class unit produced by the expression.