🚀 go-pugleaf

Bjorn's article reminds me on the following problem : In a lazy
language we can define OR in two possible ways, s.t. it has the
following behaviours : (_|_ = bottom)

OR1 True _|_  = True        OR2 True _|_  = _|_
OR1 _|_  True = _|_         OR2 _|_  True = True
...                         ...

but there is no way to get the 'intended' behaviour :

OR True _|_  = True
OR _|_  True = True
...

It is fairly obvious, that we need some sort of parallel evaluation to
get the desired result. But even given that, it's not obvious what
programming language construct would enable us to write 'symmetric'
programs like OR. Candidates are the 'fair bottom avoiding merge' or
more fundamently McCarthy's ambiguity operator [which applied to two
arguments return's a value, if the evaluation of at least one of them
terminates]. But both obviously destroy the Church-Rosser-Property !

Thorsten


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Thorsten Altenkirch 
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