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Error-triple

A common ACL2 programming idiom

When evaluation returns three values, where the first two are ordinary (non-stobj) objects and the third is the ACL2 state, the result may be called an ``error triple'' or ``error-triple''. If an error triple is (mv erp val state), we think of erp as an error flag and val as the returned value. By default, if the result of evaluating a top-level form is an error triple (mv erp val state), then that result is not printed if erp is non-nil or if val is the keyword :INVISIBLE, and otherwise val is printed with a preceding space. For example:

ACL2 !>(+ 3 4) ; ordinary value
7
ACL2 !>(mv nil (+ 3 4) state) ; error triple, error component of nil
 7
ACL2 !>(mv t (+ 3 4) state) ; error triple, non-nil error component
ACL2 !>(mv nil :invisible state) ; special case for :INVISIBLE
ACL2 !>

See programming-with-state for a discussion of error triples and how to program with them. Also see ld-error-triples and see ld for a discussion of the value :COMMAND-CONVENTIONS for keyword :LD-POST-EVAL-PRINT.