Capture Failure

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) =
  if s then
    !|[ true ]|
  else
    !|[ false ]|
  end

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

(Note that these examples use concrete syntax.)

-- EelcoVisser - 17 Jun 2002