Finally, in the upcoming release of Whiley, I have added support for both while and for loops — which is about time. Here’s an example:
define nat as int where $ >= 0
nat sum([nat] list):
r=0
i=0
while i < |list| where r >= 0:
r = r + list[i]
i = i + 1
return r
This code sums a list of [[natural number|natural numbers]] which, of course, produces a natural number. It illustrates a few points: firstly, the syntax for while loops is as expected; secondly, while loops may be given [[loop invariant|loop invariants]] (i.e. the where clause).
We can rewrite this example using a for loop as follows:
define nat as int where $ >= 0
nat sum([nat] list):
r=0
for l in list where r >= 0:
r = r + l
return r
This does what you’d expect and serves to illustrate that loop invariants can be given to for loops as well.