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Pre- and postconditions made easy

This egg implements some routines, which are outsourced from simple-exceptions. In particular macros << and >>, which accept an argument or result, checks it against zero or more predicates and returns it in case of success unchanged. Otherwise it prints a meaningful error message, showing i.a. the offending predicate and the argument's or result's name. Some are implemented as macros instead of procedures, because I didn't want an extra parameter with the argument's or result's name. Procedure versions of those macros are given as well. In reimplementing those routines, I changed the syntax a bit, so be careful, if you used the equally named routines from simple-expressions.

Interface

The precondition and postcondition checks are denoted with some consecutive symbols < and > respectively. There are macro and procedure versions, the latter denoted with a trailing % and needing an additional parameter, the name of the value to be checked. All those routines work the same, they differ only in the error message, they produce in case some predicate returns #f. The routines named with three symbols < or > differ from those with two only by an additional parameter naming the location of the checks.

assert*

[syntax] (assert* loc xpr ....)

checks the expressions xpr .... in sequence and raises an exception for the first failing expression with location property loc and arguments property the failing expression quoted.

named-lambda

[syntax] (named-lambda (name . args) xpr . xprs)

a named version of lambda, which makes recursion possible and helps in error messages.

<<<

[syntax] (<<< loc arg arg? ...)

precondition test: returns arg unchanged only if all predicates arg? return #t on it, otherwise an error message with the offending predicate and the location loc is printed.

<<

[syntax] (<< arg arg? ...)

precondition test: returns arg unchanged only if all predicates arg? return #t on it, otherwise an error message with the offending predicate is printed.

>>>

[syntax] (>>> loc result result? ...)

postcondition test: returns result unchanged only if all predicates result? return #t on it, otherwise an error message with the offending predicate and the location loc is printed.

>>

[syntax] (>> result result? ...)

postcondition test: passes result unchanged only if all predicates result? return #t on it otherwise an error message with the offending predicate is printed.

<<<%

[procedure] (<<<% loc name arg arg? ...)

precondition test: returns arg unchanged only if all predicates arg? return #t on it, otherwise an error message with the offending predicate and the location loc as well as arg's name is printed.

<<%

[procedure] (<<% name arg arg? ...)

precondition test: returns arg unchanged only if all predicates arg? return #t on it, otherwise an error message with the offending predicate as well as arg's name is printed.

>>>%

[procedure] (>>>% loc name result result? ...)

postcondition test: returns result unchanged only if all predicates result? return #t on it, otherwise an error message with the offending predicate and the location loc as well as result's name is printed.

>>%

[procedure] (>>% name result result? ...)

postcondition test: passes result unchanged only if all predicates result? return #t on it otherwise an error message with the offending predicate as well as result's name is printed.

true?

[procedure] (true? xpr)

returns always #t

false?

[procedure] (false? xpr)

returns always #f

Examples

(define x 5)

(<< x positive? odd?)

-> 5

(<< x zero? odd?)

-> error

(<< x positive? even?)

-> error

(<<< 'loc x positive? even?)

-> error message with predicate even?, location 'loc and name 'x

(<<<% 'loc 'x x positive? even?)

-> error message with predicate even?, location 'loc and name 'x

Last update

Mar 27, 2019

Author

Juergen Lorenz

License

 
Copyright (c) 2014-2019, Juergen Lorenz
All rights reserved.

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Version History
1.2
code cleaned
1.1.0
documentation of named-lambda fixed
1.1
procedure versions added
1.0
extracted and modified from simple-exceptions