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continuations

This module provides some syntactic sugar to Marc Feeley's continuation interface. In this interface, continuations are a datatype separate from procedures. Hence it provides a continuation? predicate. I've stripped the prefix from the procedures continuation-capture and continuation-graft and renamed continuation-return throw. This latter procedure is accompanied in this module by a macro named catch, so that the usual catch-throw-semantics of other languages is available. For example, a typical use pattern of this module is as follows

 
(catch k
  ...
  (if ...
    (throw k val)
    ...))

But note, that in this pattern, k is a Scheme object of type continuation, and like every Scheme object it has indefinite extent, hence can be exported, saved in other data-structures etc. So this pattern is much more powerful than the corresponding pattern in other languages.

Two other procedures are provided as well, continuation->procedure, which does what the name says, and continuations, which provides offline documentation to the module. Like in modules written in the design-by-contract style, the call (continuations) lists the exported symbols, and (continuations sym) provides documentation of the exported symbol sym.

The routines

continuations

[procedure] (continuations [sym])

documentation procedure

continuation?

[procedure] (continuation? xpr)

type predicate

continuation->procedure

[procedure] (continuation->procedure cont)

transforms a continuation into a procedure

capture

[procedure] (capture proc)

The same as call/cc but with a different datatype: Captures the current continuation as a continuation datatype (contrary to a procedure datatype in call/cc) and calls proc with that continuation as its only argument.

graft

[procedure] (graft cont thunk)

tail-calls thunk with the implicit continuation cont.

throw

[procedure] (throw cont . vals)

throws the values vals to the continuation cont.

catch

[syntax] (catch cont xpr . xprs)

The same as let/cc of miscmacros but with a different datatype: Binds the cont variable to the current continuation as a continuation and executes the body xpr . xprs in this context.

Requirements

none

Last update

May 25, 2013

Author

Juergen Lorenz

License

 
Copyright (c) 2013, Juergen Lorenz
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Version History
1.0
initial import