Describing a String type in Ada

Question

I have a type similar to:

type ID is new String (1 .. 7);
-- Example: 123-456

How can I specify that format in code, either with Ada or SPARK?

I was thinking about Static_Predicate, but the condition that the string must start with 3 positive integers followed by a dash followed by another set of 3 positive integers can't be described with a Static_Predicate expression.

score 5 · Accepted Answer · edited Nov 06 '17 at 05:44

5

You have to use a Dynamic_Predicate for this:

type ID is new String (1 .. 7)
  with Dynamic_Predicate => (for all I in ID'Range =>
                               (case I is
                                   when 1 .. 3 | 5 .. 7 => ID (I) in '0' .. '9',
                                   when 4               => ID (I) in '-'));

I'm using this quite a bit myself, but I mostly make the types subtypes of String instead of actual new types.

edited Nov 06 '17 at 05:44

rid

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answered Nov 05 '17 at 15:21

Jacob Sparre Andersen

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When I try that, I get the error: "of" should be "is". – rid Nov 05 '17 at 16:30
I _think_ GNAT lets you use just `Predicate` (it works out whether to use `Dynamic_Predicate` or `Static_Predicate` for itself) – Simon Wright Nov 05 '17 at 17:10
Messed up. Corrected now. – Jacob Sparre Andersen Nov 05 '17 at 18:53
@SimonWright, but that is GNAT specific behaviour, and unlikely to get into the standard, as it makes it unclear if a subtype is valid for use in a case statement or not. – Jacob Sparre Andersen Nov 05 '17 at 18:54
Great, thanks. it works. I created a separate validation function and used that as the `Dynamic_Predicate`. – rid Nov 06 '17 at 05:50

Describing a String type in Ada

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