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added a ``dissection'' of a contract error message to the Guide

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closes PR 12738
This commit is contained in:
Robby Findler 2012-05-16 08:50:25 -05:00
parent 67c48d6d78
commit fc3f4e675f
2 changed files with 35 additions and 16 deletions
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16
collects/scribblings/guide/contracts-intro.scrbl
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@ -181,20 +181,6 @@ the module boundary for a second time.
</question> </question>
} }
@;{
@ctc-section[#:tag "obligations"]{Imposing Obligations on a Module's Clients}
On occasion, a module may want to enter a contract with
another module only if the other module abides by certain
rules. In other words, the module isn't just promising some
services, it also demands the client to deliver
something. That situation may happen when a module exports
a function, an object, a class, or some other construct that enables
values to flow in both directions.
}
@ctc-section{Experimenting with Contracts and Modules} @ctc-section{Experimenting with Contracts and Modules}
All of the contracts and modules in this chapter (excluding those just All of the contracts and modules in this chapter (excluding those just
@ -203,7 +189,7 @@ describing modules. Since modules serve as the boundary between
parties in a contract, examples involve multiple modules. parties in a contract, examples involve multiple modules.
To experiment with multiple modules within a single module or within To experiment with multiple modules within a single module or within
DrRacket's @tech{definitions area}, use the DrRacket's @tech{definitions area}, use
Racket's submodules. For example, try the example earlier in Racket's submodules. For example, try the example earlier in
this section like this: this section like this:

35
collects/scribblings/guide/contracts-simple-function.scrbl
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@ -1,5 +1,8 @@
#lang scribble/doc #lang scribble/doc
@(require scribble/manual scribble/eval "guide-utils.rkt" "contracts-utils.rkt" @(require scribble/manual scribble/eval
scribble/core racket/list
scribble/racket
"guide-utils.rkt" "contracts-utils.rkt"
(for-label racket/contract)) (for-label racket/contract))
@title[#:tag "contract-func"]{Simple Contracts on Functions} @title[#:tag "contract-func"]{Simple Contracts on Functions}
@ -385,3 +388,33 @@ With this little change, the error message becomes quite readable:
contract-eval contract-eval
(require 'improved-bank-server) (require 'improved-bank-server)
(deposit -10)] (deposit -10)]
@; not sure why, but if I define str directly to be the
@; expression below, then it gets evaluated before the
@; expressions above it.
@(define str "huh?")
@(begin
(set! str
(with-handlers ((exn:fail? exn-message))
(contract-eval '(deposit -10))))
"")
@ctc-section[#:tag "dissecting-contract-errors"]{Dissecting a contract error message}
@(define (lines a b)
(define lines (regexp-split #rx"\n" str))
(table (style #f '())
(map (λ (x) (list (paragraph error-color x)))
(take (drop lines a) b))))
In general, each contract error message consists of six sections:
@itemize[@item{a name for the function or method associated with the contract
and either the phrase ``contract violation'' or ``violated it's contract''
depending on whether the contract was violated by the server or the
client; e.g. in the previous example: @lines[0 1]}
@item{a description of the precise aspect of the contract that was violated, @lines[1 1]}
@item{the complete contract plus a path into it showing which aspect was violated, @lines[2 2]}
@item{the module where the contract was put (or, more generally, the boundary that the contract mediates), @lines[4 1]}
@item{who was blamed, @lines[5 1]}
@item{and the source location where the contract appears. @lines[6 1]}]