The predicate language allows the user to define simple predicates (Boolean-valued functions) that Dune can evaluate. Here is a semiformal specification of the predicate language:
The exact meaning of
:standard and the nature of
element depend on
the context. For example, in the case of the dirs, an
element corresponds to file glob patterns. Another example is the user
(with-accepted-exit-codes), where an
corresponds to a literal integer.