Thomas Hafner
1/27/2009 8:44:00 PM
Hello,
in his book ``The Ruby Programming Language'' Mats discourages from
using continuations. Nevertheless I've found a real world example,
where at a first look callcc could be easily replaced by catch and
throw, but indeed it's a little bit problematic:
#!/usr/bin/env ruby
# -*-mode: ruby; coding: utf-8;-*-
require 'jcode'
require 'pp'
$-K='U'
module MultiLine
def transform( input )
document( input, 0 ){ |p, value| value.flatten }
end
private
def document( input, p, acc = [] )
loop {
callcc { |proceed|
return yield( p, acc ) unless input[p]
multiline( input, p ){ |p0, v0|
p = p0
acc << v0
proceed.call
}
character( input, p ){ |p1, v1|
p = p1
acc << v1
proceed.call
}
return nil
}
}
end
def multiline( input, p )
'(' == input[p] && multiline_tail( input, p+1 ){ |p2, v2|
return yield( p2, v2 )
}
end
def multiline_tail( input, p, acc=[] )
loop {
callcc { |proceed|
case input[p]
when nil
return nil
when ')'
return yield( p+1, acc )
when "\n", "\r"
p += 1
acc << ' '
proceed.call
end
multiline( input, p ){ |p2, v2|
p = p2
acc << v2
proceed.call
}
character( input, p ){ |p4, v4|
p = p4
acc << v4
proceed.call
}
return nil
}
}
end
def character( input, p )
case input[p]
when nil
return nil
when '\\'
case input[p+1]
when nil
return nil
when '\\', '(', ')'
return yield( p+2, input[p+1] )
end
end
return yield( p+1, input[p] )
end
extend( self )
end
if __FILE__ == $0
input = STDIN.read.split(//)
output = MultiLine.transform( input )
print output
end
The problem is, that the methods can be called recursively, and
catching from a throw should continue at the correct recursion level.
No symbol that has to be thrown/cached should be reused. How about
providing each ``catch'' with an individual symbol?:
# ...
def document( input, p, acc = [] )
proceed = gensym
loop { catch proceed do
return yield( p, acc ) unless input[p]
multiline( input, p ){ |p0, v0|
p = p0
acc << v0
throw proceed
}
character( input, p ){ |p1, v1|
p = p1
acc << v1
throw proceed
}
return nil
end }
end
# ... a.s.o. ...
def multiline_tail( input, p, acc=[] )
proceed = gensym
loop { catch proceed do
case input[p]
when nil
return nil
when ')'
return yield( p+1, acc )
when "\n", "\r"
p += 1
acc << ' '
throw proceed
end
multiline( input, p ){ |p2, v2|
p = p2
acc << v2
throw proceed
}
character( input, p ){ |p4, v4|
p = p4
acc << v4
throw proceed
}
return nil
end }
end
# ... a.s.o. ...
A new method is needed to generate the unique symbols:
def gensym
[].object_id.to_s.to_sym
end
The catch/throw solution seems uglier to me than the callcc solution.
Do you have other suggestions?
Regards
Thomas