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

  ConstFold :
    [[ i < j ]] -> <lt>(i, j)

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.

  ConstFold :
    [[ i < j ]] -> <to-bool(<lt>(i,j))>

where to-bool could be defined as

  to-bool(s) =
    s; ![[ true ]] <+ ![[ false ]]

that is, when s succeeds return true, otherwise return false.

(Note that these examples use concrete syntax.)

-- EelcoVisser - 17 Jun 2002