Folded prerequisites

A note on checkers

Checkers can be used as both expected values of assertions and also in the argument lists of prerequisites:

    (fact
      (f 1) => truthy   ; as expected value of assertion
      (provided 
         (g truthy) => 3))   ; as argument of prerequisite

Some checkers, like roughly, are actually functions that generate checking functions:

    (fact
      (f 1) => (exactly odd?)
      (provided
         (g (roughly 3.0)) => 800.0))

How does Midje know that (g (roughly ...)) is a checker, not a prerequisite to be folded? It is defined specially:

     (defchecker roughly
       "With two arguments, accepts a value within delta of the
        expected value. With one argument, the delta is 1/1000th
        of the expected value."
       ([expected delta]
         (checker [actual]  ; <<= Checker tags the generated function as a checker.
           (and (>= expected (- actual delta))
                (<= expected (+ actual delta)))))
       ([expected]
         (roughly expected (* 0.001 expected))))

If you define your own function-generating checkers, they should use similar macros. See Checkers within prerequisites.