Many common higher-order functions consume a function value as the first argument, and n more arguments after that, where the function value accepts n arguments, which corresponds to the arguments in the call in some way. Examples include: map, filter (only one argument), andmap, ormap (foldl and foldr have arguments in a wrong order, so they don't quite work)
Example code without pyret-for
(define things '(("pen") ("pineapple") ("apple") ("pen")))
(define quantities '(1 2 3 5))
(andmap (λ (thing quantity)
(or (string-contains? (first thing) "apple")
(odd? quantity)))
things
quantities)
The problem is that
- It's difficult for readers to relate formal arguments of the function value to the actual arguments of the call.
- There's a lot of rightward drift.
Example code with pyret-for
(define things '(("pen") ("pineapple") ("apple") ("pen")))
(define quantities '(1 2 3 5))
(pyret-for andmap ([thing things] [quantity quantities])
(or (string-contains? (first thing) "apple")
(odd? quantity)))
The pyret-for syntax, based on Pyret's for, can be used to invoke this kind of higher-order functions.
pyret-for additionally improves upon Pyret's for by allowing arbitrary match pattern.
(define things '(("pen") ("pineapple") ("apple") ("pen")))
(define quantities '(1 2 3 5))
(pyret-for andmap ([(list thing) things] [quantity quantities])
(or (string-contains? thing "apple")
(odd? quantity)))
Macro
(require syntax/parse/define)
(define-syntax-parse-rule (pyret-for f:expr ([pat:expr arg:expr] ...) body:expr ...+)
#:with (x ...) (generate-temporaries (attribute arg))
(f (λ (x ...)
(match-define pat x) ...
body ...)
arg ...))
Licence
MIT License / CC BY 4.0