8.12
2 Prisms
| (require glass/prism) | package: glass |
A prism is a type of optic for focusing on a specific kind of subject and ignoring other kinds. A prism is built from a matching function, which focuses on subjects of the correct kind, and a casting function, which transforms a replacement focus back into the subject.
A predicate for prisms.
A contract combinator for prisms. Creates a contract
that accepts prisms whose subjects are checked with subject-contract
and whose foci are checked with focus-contract.
procedure
(make-prism matcher caster [#:name name]) → prism?
matcher : (-> any/c option?) caster : (-> any/c any/c) name : (or/c interned-symbol? #f) = #f
Constructs a prism named name.
Examples:
(define number-string-prism (make-prism (λ (s) (define num (string->number s)) (if num (present num) absent)) number->string #:name 'number-string-prism))
> (prism-match number-string-prism "124") (present 124)
> (prism-match number-string-prism "elephant") #<absent>
> (prism-cast number-string-prism 100) "100"
procedure
(prism-match prism subject) → option?
prism : prism? subject : any/c
Matches subject against prism, returning an
option that is present if prism was able to extract a
focus from subject and absent otherwise.
Examples:
> (prism-match success-prism (success 123)) (present 123)
> (prism-match success-prism (failure "oh no!")) #<absent>
procedure
(prism-cast prism focus) → any/c
prism : prism? focus : any/c
Casts focus back into a subject value using prism. This is
the inverse operation of prism-match —
Example:
> (prism-cast success-prism 123) (success 123)
value
value
Examples:
> (prism-match success-prism (failure "kaboom!")) #<absent>
> (prism-match failure-prism (failure "kaboom!")) (present "kaboom!")
> (prism-cast failure-prism "kaboom!") (failure "kaboom!")