Predictions

A prediction has three parts: a left-hand side, an arrow, and a right-hand side. With the normal => arrow, the left-hand side and right-hand side are evaluated, then the results are compared using extended equality. Extended equality is by default =, but the type of the right (and sometimes the left) side can change that. By far the two most important cases are:

• functions

  A function on the right-hand side is given the result of evaluating the left-hand side. If the result is any value that Clojure counts as true, the prediction succeeds.

• regular expressions

  This prediction:

  (f) => #"foo+"

  ... means the same thing as this:

  (re-find #"foo+" (f)) => truthy

  (truthy is a checker that succeeds when given any value that Clojure counts as true.)

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=> is not the only kind of arrow. Other arrows behave differently.

• =not=> or =deny=>

  The claim checks out only if extended equality does not produce a truthy value.

  (fact (nth-prime 100) =not=> even?)

• =expands-to=>

  The left-hand side is macroexpanded and compared to the right-hand side:

  (fact (when true 3) =expands-to=> (if true (do 3)))

  Note that neither side is quoted.

• =future=>

  Acts like a future fact in that it produces a "WORK TO DO" message. This is useful when you've got most of the claims in a fact working, but have to defer finishing one or a few.