🚀 go-pugleaf

In article <7044@ganymede.inmos.co.uk> geoffb@butcombe.inmos.co.uk (Geoff
Barrett) writes:
%I believe there is an Oxford DPhil thesis by a man called David Turner
%which pointed out that strict languages are not referentially
%transparent because of the fact that conditionals are not strict, so
%that the equality symbol in the definition

%	fun iiff (b,x,y) = if b then x else y

%is not equality.  Lazy evaluation languages are referentially
%transparent.  I think this is what the original poster meant by
%"purer".

Hmmm. How would a lazy language treat a call

if(b,x,x)

where the evaluation of b is non-terminating but the evaluation of x
terminates? If "naive" left-to-right evaluation order is assumed, the
computation will not terminate. On the other hand, it is reasonable to
define the semantics of the if-function so that such a call returns the
value of x. (After all, the value of x will be the answer regardless of what
b evaluates to). What about "referential transparency" here?

This problem is akin to left-to-right evaluation sensitivity of goals in
Prolog; the order of goals that are ANDed together in a Horn clause can
affect termination even though AND, considered as a "mathematical" binary
operation, is commutative.

Bjorn Lisper