[Why3-club] Non-int equality

Kruer, Joseph C. jkruer at draper.com
Wed Jul 18 21:19:47 CEST 2018

Hello Why3-ople,

I'm working on a verification task working with strings and therefore find myself doing lots of individual character comparisons. I ran into an error that left me scratching my head for a bit in something like the following code:

let headIsA (s: list char) : bool =
    match HdTl.hd s with
    | Some(c) -> c = Char.chr 41
    | None -> False

This causes Why3 to raise the following error: "This expression has type char, but is expected to have type int". I'm fairly certain now that this is because (=) is defined in the Int module as a binary operation on ints. My initial solution to this was to explicitly use c.code (which is a private field of the record type char) and compare against the integer value of the character in question. Another idea I had that felt a bit nicer was to define a pure function (=) taking a pairs of ints to bools, but this raised another error: "Logical equality cannot be redefined." Finally I defined it as a method with an ensures clause like Int's (=) which did the trick.

My lingering questions at this point are essentially these:

1.       Does my solution of overloading (=) to operate on chars break anything? Could I add it to the stdlib's Char module?

2.       How does "logical equality" work? is it polymorphic?

3.       How do the logical/non-logical "namespaces" of Why3 work?

4.       Could one add to Why3 a polymorphic equality operator for record types that might be defined as, say, field-wise equality?

- Jay

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