On this page:
visit
visitor?
visitor-combine
make-default-visitor
make-leaf-visitor
make-visitor
make-wrapper-visitor
make-uniform-default-visitor
make-uniform-leaf-visitor
make-uniform-visitor
make-uniform-wrapper-visitor
map-visitor
for-each-visitor
map/  reduce-visitor
make-map-visitor
make-for-each-visitor
make-map/  reduce-visitor
make-memoizing-visitor
7.1

7.7 mischief/visitor: Compositional, Higher-Order Value Traversals

 (require mischief/visitor) package: mischief

procedure

(visit visitor target arg-or-keyword-arg ...)  any

  visitor : visitor?
  target : any/c
  arg-or-keyword-arg : any/c
Invokes visitor on target, passing on the given arg-or-keyword-args.

procedure

(visitor? x)  boolean?

  x : any/c
Recognizes visitors.

procedure

(visitor-combine visitor ...)  visitor?

  visitor : visitor?
Constructs a compound visitor that tries the given visitors from left to right to find one that can process any given value.

procedure

(make-default-visitor handler)  visitor?

  handler : (-> any/c any/c ... any/c)
Constructs a visitor that always calls handler with the target value and any additional arguments.

procedure

(make-leaf-visitor predicate handler)  visitor?

  predicate : (-> any/c boolean?)
  handler : (-> any/c any/c ... any/c)
Constructs a visitor that handles target values that satisfy predicate. These target values are passed to handler along with any additional arguments that were passed to the visitor.

procedure

(make-visitor predicate handler)  visitor?

  predicate : (-> any/c boolean?)
  handler : (-> (-> any/c any/c ... any/c) any/c any/c ... any/c)
Constructs a visitor that handles target values that satisfy predicate. The handler is called with a recursive handler procedure, the target value, and any additional arguments that were passed to the visitor.

procedure

(make-wrapper-visitor handler)  visitor?

  handler : 
(-> (-> any/c any/c ... any/c)
    (-> any/c any/c ... any/c)
    any/c any/c ... any/c)
Constructs a visitor that processes all target values, given two recursive handler procedures. The first resumes the search for a relevant visitor after the current one within a compound visitor, and is usually used for the given value when the current visitor is not relevant. The second uses the whole visitor in use, and is intended for recursive handling of sub-parts.

procedure

(make-uniform-default-visitor handler)  visitor?

  handler : (-> any/c any/c)

procedure

(make-uniform-leaf-visitor predicate    
  handler)  visitor?
  predicate : (-> any/c boolean?)
  handler : (-> any/c any/c)

procedure

(make-uniform-visitor predicate handler)  visitor?

  predicate : (-> any/c boolean?)
  handler : (-> (-> any/c any/c) any/c)

procedure

(make-uniform-wrapper-visitor handler)  visitor?

  handler : (-> (-> any/c any/c) (-> any/c any/c) any/c any/c)
Variants of make-default-visitor, make-leaf-visitor, make-visitor, and make-wrapper-visitor that ignore any additional arguments, and automatically pass them on unchange to all recursive visitor calls.

A visitor that reconstructs its target value.

A visitor that executes side effects for all parts of its target value, and always produces (void).

A visitor that expects two additional arguments: a unary map function to transform individual elements of the target value, and a variable-arity reduce function to combine results.

procedure

(make-map-visitor predicate    
  constructor    
  accessor ...)  visitor?
  predicate : (-> any/c boolean?)
  constructor : (-> any/c ... predicate)
  accessor : (-> predicate any/c)
Creates a visitor that handles values satisfying predicate by extracting their fields with the given accessors, recursively visiting each, then reassembling the result using the constructor.

procedure

(make-for-each-visitor predicate    
  accessor ...)  visitor?
  predicate : (-> any/c boolean?)
  accessor : (-> predicate/c any/c)
Creates a visitor that handles values satisfying predicate by extracting their fields with the given accessors and recursively visiting each. Always returns (void).

procedure

(make-map/reduce-visitor predicate    
  accessor ...)  visitor?
  predicate : (-> any/c boolean?)
  accessor : (-> predicate/c any/c)
Creates a visitor that handles values satisfying predicate by extracting their fields with the given accessors, recursively visiting each, then reassembling the result using the visitor’s reduce function.

procedure

(make-memoizing-visitor [memo])  visitor?

  memo : memo-table? = (make-memo-table)
Creates a visitor that memoizes the results of passing each target value to subsequent visitors.