"Implies" matcher

[xtreme-james-cooper]

A simple implies matcher, for use in invariants [#294, #289]. Because in an invariant, unlike a regular hand-written unit test, the exact values of the variables at test-time may not be known, it may be necessary to add in conditionals to ensure the perpetual truth of the invariant:

invariant(@"should display the appropriate popup on purchase-click", ^{
    [subject.payNowButton sendActionsForControlEvent:UIControlEventTouchUpInside];
    if (! [AlertviewHelper isViewControllerDisplayingAlertView: subject]) {
        subject.confirmDialogue.displayed should be_truthy;
    }
});

Adding in the implies logical connective as a matcher makes this simpler and more declarative:

invariant(@"should display the appropriate popup on purchase-click", ^{
    [subject.payNowButton sendActionsForControlEvent:UIControlEventTouchUpInside];

    BOOL clickPossible = ! [AlertviewHelper isViewControllerDisplayingAlertView: subject];
    clickPossible should imply(subject.confirmDialogue.displayed);
});

The implies matcher follows the standard logical connective semantics:

X	Y	X implies Y
T	T	T
T	F	F
F	T	T
F	F	T

Currently it only supports boolean arguments (or object arguments interpreted as nil/not nil). One possible improvement would be to upgrade it to a full-blown matcher-combinator, taking matchers as one or either argument, enabling the user to leverage the full scope of matchers available; although most matchers can be implemented reasonably clearly and efficiently as a boolean test (X should be_instance_of(Y) as [x isKindOfClass: Y], for instance) so this doesn't seem as critical, and it would open the door to nested matcher combinators etc, which is probably not the road to go down just yet.

[xtreme-james-cooper]

A different use case is checking "homemade algebraic data types": the common ObjC design pattern where a field is only valid if a flag is set.

data MyData = NoFields | OneField [Int] | TwoFields Int String 

might be represented as

typedef enum {
    NO_FIELDS,
    ONE_FIELD,
    TWO_FIELDS
} MyData;
...
@property (nonatomic, assign) MyData myDataFlag;
@property (nonatomic, assign) NSArray* intArrayField;
@property (nonatomic, assign) int intField;
@property (nonatomic, assign) NSString* stringField;

With an implies matcher, the invariant for this can be easily checked:

invariant(@"MyData should behave like the ADT it's emulating", ^{
    (subject.myDataFlag == NO_FIELDS) should imply (subject.intArrayField == nil);
    (subject.myDataFlag == NO_FIELDS) should imply (subject.intField == 0);
    (subject.myDataFlag == NO_FIELDS) should imply (subject.stringField == nil);

    (subject.myDataFlag == ONE_FIELD) should imply (subject.intArrayField != nil);
    (subject.myDataFlag == ONE_FIELD) should imply (subject.intField == 0);
    (subject.myDataFlag == ONE_FIELD) should imply (subject.stringField == nil);

    (subject.myDataFlag == TWO_FIELDS) should imply (subject.intArrayField == nil);
    (subject.myDataFlag == TWO_FIELDS) should imply (subject.intField != 0);
    (subject.myDataFlag == TWO_FIELDS) should imply (subject.stringField != nil);
});