Success and failure in Stratego allows one to avoid computing with Boolean values all the time. However,  sometimes it is necessary to capture the failure (or success) of a strategy and translate it into a term. For example, when performing [[constant folding]] of relational expressions one would like to write
<verbatim>
ConstFold :
  |[ i < j ]| -> <lt>(i, j)
</verbatim>
However, when the =lt= fails, the rule fails. What is needed is a little wrapper strategy that turns failure
and success into an appropriate term.
<verbatim>
ConstFold :
  |[ i < j ]| -> <to-bool(<lt>(i,j))>
</verbatim>
where =to-bool= could be defined as
<verbatim>
to-bool(s) =
  if s then
	 !|[ true ]|
  else
	 !|[ false ]|
  end
</verbatim>
that is, when =s= succeeds return =true=, otherwise return =false=.

(Note that these examples use [[concrete syntax]].)

-- Main.EelcoVisser - 17 Jun 2002 <br />