Ken, the purpose of this is a little different than the usual data step.

Our data step is being used to check if a user supplied expression is valid in an IF statement. If an error is detected in the data step then this indicates an error in the expression. The problem is that type mismatches are not flagged as errors by SAS but we want them to be considered invalid. The conversion is done automatically by SAS and there is only a note in the log.

Testing the expression in a WHERE statement would work better because WHERE statements (unlike IF statements) do not allow type mismatches. The problem with testing with WHERE is that it allows a number of special expressions that the IF statement does not allow.