After a strategy fails, variables that have been bound in matches should be unbound. This
is not supported by the current compilation scheme. 
The following example illustrates the problem. The strategy =f= should behave the same
as the choice between =g= and =h=.

<verbatim>
module unbind-on-backtrack 
strategies  
  f = (?(x, y); ?(x, x) <+ ?(y, x)); ![y,x]  

  g = ?(x, y); ?(x, x); ![y,x] 
 
  h = ?(y, x); ![y,x]  

  main = 
	 <g <+ h> ("a", "b") => ["a", "b"]
  ; <f> ("a", "b") => ["a", "b"]					
</verbatim>

-- Main.EelcoVisser - 22 Nov 2001 <br />

One way: only bind unbound variables when choice is committed:
Consider
<verbatim>
  ?F(x, y); s1 <+ s2
</verbatim>
and assume that x is bound before invocation of this strategy and
that y is not (y value is looked for). transform this into
<verbatim>
  {y: ?F(x, y); s1; split(id,!y)}; ?(_,y); Fst <+ s2
</verbatim>
This has the same effect as the earlier expression (can this insight
be made more efficient? turned into primitive operations?)

Note: this assume one can determine statically which variables are
unbound when entering a choice. Although this scheme works when y
is bound, it might turn out very expensive, because a failing match
to y is discovered after doing s1.

-- Main.EelcoVisser - 2000

Solution: Save (the status of) all variables that are bound in the left branch of
a choice and restore after failure. Shouldn't be too difficult.

-- Main.EelcoVisser - 22 Nov 2001 <br />

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CategoryBugs