87 lines
2.9 KiB
Racket
87 lines
2.9 KiB
Racket
#lang scribble/doc
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@(require "web-server.ss")
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@title[#:tag "web-cells.ss"]{Web Cells}
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@(require (for-label web-server/servlet/web-cells
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web-server/servlet/web))
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@defmodule[web-server/servlet/web-cells]{The
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@schememodname[web-server/servlet/web-cells] library provides the
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interface to Web cells.
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A Web cell is a kind of state defined relative to the @defterm{frame tree}.
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The frame-tree is a mirror of the user's browsing session. Every time a
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continuation is invoked, a new frame (called the @defterm{current frame}) is
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created as a child of the current frame when the continuation was captured.
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You should use Web cells if you want an effect to be encapsulated in all
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interactions linked from (in a transitive sense) the HTTP response being
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generated. For more information on their semantics, consult the paper
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@href-link["http://www.cs.brown.edu/~sk/Publications/Papers/Published/mk-int-safe-state-web/"
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"\"Interaction-Safe State for the Web\""].
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@defproc[(web-cell? [v any/c])
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boolean?]{
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Determines if @scheme[v] is a web-cell.
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}
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@defproc[(make-web-cell [v any/c])
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web-cell?]{
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Creates a web-cell with a default value of @scheme[v].
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}
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@defproc[(web-cell-ref [wc web-cell?])
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any/c]{
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Looks up the value of @scheme[wc] found in the nearest
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frame.
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}
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@defproc[(web-cell-shadow [wc web-cell?]
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[v any/c])
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void]{
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Binds @scheme[wc] to @scheme[v] in the current frame, shadowing any
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other bindings to @scheme[wc] in the current frame.
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}
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Below is an extended example that demonstrates how Web cells allow
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the creation of reusable Web abstractions without requiring global
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transformations of the program into continuation or store passing style.
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@schememod[
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web-server/insta
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(define (start initial-request)
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(define counter1 (make-counter))
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(define counter2 (make-counter))
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(define include1 (include-counter counter1))
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(define include2 (include-counter counter2))
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(send/suspend/dispatch
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(lambda (embed/url)
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`(html
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(body (h2 "Double Counters")
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(div (h3 "First")
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,(include1 embed/url))
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(div (h3 "Second")
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,(include2 embed/url)))))))
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(define (make-counter)
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(make-web-cell 0))
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(define (include-counter a-counter)
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(let/cc k
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(let loop ()
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(k
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(lambda (embed/url)
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`(div (h3 ,(number->string (web-cell-ref a-counter)))
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(a ([href
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,(embed/url
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(lambda _
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@code:comment{A new frame has been created}
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(define last (web-cell-ref a-counter))
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@code:comment{We can inspect the value at the parent}
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(web-cell-shadow a-counter (add1 last))
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@code:comment{The new frame has been modified}
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(loop)))])
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"+")))))))
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]
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}
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