Translating Eiffel loops to languages that do not support loop invariants/variants

Question

A loop in Eiffel follows this format:

from
    Init
invariant
    Invariant
until
    Exit
variant
    Variant
loop
    Body
end

How would you translate the above Eiffel pseudo-code to a language that does not support loop invariants/variants? Let's assume that such target language has an assert instruction to check the invariant/variant.

score 2 · Accepted Answer · answered Mar 17 '16 at 13:21

2

It would look like that:

Init
last := infinity
loop
    assert (Invariant)
    next := Variant
    assert (0 <= next and next < last)
    last := next
    if Exit then
        break
    end
    Body
end

answered Mar 17 '16 at 13:21

Alexander Kogtenkov

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Excuse me: shouldn't `Variant` be a predicate like `Invariant`? – Eleno Mar 17 '16 at 14:00
@Elena, `Variant` is an expression of an integer type. There is a tag associated with it like with other assertions, but unlike other assertions it is not a Boolean. – Alexander Kogtenkov Mar 17 '16 at 14:03
So basically Eiffel checks that `Variant` decreases at each iteration, while staying non-negative, right? – Eleno Mar 17 '16 at 14:36
@Elena, yes, this is the idea to make sure the loop terminates. – Alexander Kogtenkov Mar 17 '16 at 19:00

Translating Eiffel loops to languages that do not support loop invariants/variants

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