Assertions in design by contract
According to Wikipedia; Assertions can function as a form of documentation: they can describe the state the code expects to find before it runs (its preconditions), and the state the code expects to result in when it is finished running (postconditions); they can also specify invariants of a class.
Show in the program language of your choice an example of the expecting results as a form of documentation.
Scala
The last line is only executed if all assertions are met.<lang scala>import java.net.{URLDecoder, URLEncoder}
import scala.compat.Platform.currentTime
object UrlCoded extends App {
val original = """http://foo bar/""" val encoded: String = URLEncoder.encode(original, "UTF-8")
assert(encoded == "http%3A%2F%2Ffoo+bar%2F", s"Original: $original not properly encoded: $encoded")
val percentEncoding = encoded.replace("+", "%20") assert(percentEncoding == "http%3A%2F%2Ffoo%20bar%2F", s"Original: $original not properly percent-encoded: $percentEncoding")
assert(URLDecoder.decode(encoded, "UTF-8") == URLDecoder.decode(percentEncoding, "UTF-8"))
println(s"Successfully completed without errors. [total ${currentTime - executionStart} ms]")
}</lang>
Tcl
<lang tcl># Custom assertions; names stolen from Eiffel keywords proc require {expression message args} {
if {![uplevel 1 [list expr $expression]]} {
set msg [uplevel 1 [list format $message] $args] return -level 2 -code error "PRECONDITION FAILED: $msg"
}
} proc ensure {expression {message ""} args} {
if {![uplevel 1 [list expr $expression]]} {
set msg [uplevel 1 [list format $message] $args] return -level 2 -code error "POSTCONDITION FAILED: $msg"
}
}
proc connect_to_server {server port} {
require {$server ne ""} "server address must not be empty" require {[string is integer -strict $port]} "port must be numeric" require {$port > 0 && $port < 65536} "port must be valid for client"
set sock [socket $server $port]
# Will never fail: Tcl *actually* throws an error on connection # failure instead, but the principle still holds. ensure {$sock ne ""} "a socket should have been created"
return $sock
}</lang>
This can be usefully mixed with Tcl 8.6's try … finally …
built-in command.
- Example output from above code:
% connect_to_server "" "" PRECONDITION FAILED: server address must not be empty % connect_to_server localhost 123456 PRECONDITION FAILED: port must be valid for client