If you want to prove that the result of some tail-recursive summation function equals the result of a given recursive summation function for some value N, then it should, in principle, suffice to only define the recursive function (as an expression function) without any post-condition. You then only need to mention the recursive (expression) function in the post-condition of the tail-recursive function (note that there was no post-condition (ensures) on the recursive function in Dafny either).
However, as one of SPARK's primary goal is to proof the absence of runtime errors, you must have to prove that overflow cannot occur and for this reason, you do need a post-condition on the recursive function. A reasonable choice for such a post-condition is, as @Jeffrey Carter already suggested in the comments, the explicit summation formula for arithmetic progression:
Sum (N) = N * (1 + N) / 2
The choice is actually very attractive as with this formula we can now also functionally validate the recursive function itself against a well-known mathematically explicit expression for computing the sum of a series of natural numbers.
Unfortunately, using this formula as-is will only bring you somewhere half-way. In SPARK (and Ada as well), pre- and post-conditions are optionally executable (see also RM 11.4.2 and section 5.11.1 in the SPARK Reference Guide) and must therefore themselves be free of any runtime errors. Therefore, using the formula as-is will only allow you to prove that no overflow occurs for any positive number up until
max N s.t. N * (1 + N) <= Integer'Last <-> N = 46340
as in the post-condition, the multiplication is not allowed to overflow either (note that Natural'Last = Integer'Last = 2**31 - 1).
To work around this, you'll need to make use of the big integers package that has been introduced in the Ada 202x standard library (see also RM A.5.6; this package is already included in GNAT CE 2021 and GNAT FSF 11.2). Big integers are unbounded and computations with these integers never overflow. Using these integers, one can proof that overflow will not occur for any positive number up until
max N s.t. N * (1 + N) / 2 <= Natural'Last <-> N = 65535 = 2**16 - 1
The usage of these integers in a post-condition is illustrated in the example below.
Some final notes:
The Subprogram_Variant aspect is only needed to prove that a recursive subprogram will eventually terminate. Such a proof of termination must be requested explicitly by adding an annotation to the function (also shown in the example below and as discussed in the SPARK documentation pointed out by @egilhh in the comments). The Subprogram_Variant aspect is, however, not needed for your initial purpose: proving that the result of some tail-recursive summation function equals the result of a given recursive summation function for some value N.
To compile a program that uses functions from the new Ada 202x standard library, use compiler option -gnat2020.
While I use a subtype to constrain the range of permissible values for N, you could also use a precondition. This should not make any difference. However, in SPARK (and Ada as well), it is in general considered to be a best practise to express contraints using (sub)types as much as possible.
Consider counterexamples as possible clues rather than facts. They may or may not make sense. Counterexamples are optionally generated by some solvers and may not make sense. See also the section 7.2.6 in the SPARK user’s guide.
main.adb
with Ada.Numerics.Big_Numbers.Big_Integers;
procedure Main with SPARK_Mode is
package BI renames Ada.Numerics.Big_Numbers.Big_Integers;
use type BI.Valid_Big_Integer;
-- Conversion functions.
function To_Big (Arg : Integer) return BI.Valid_Big_Integer renames BI.To_Big_Integer;
function To_Int (Arg : BI.Valid_Big_Integer) return Integer renames BI.To_Integer;
subtype Domain is Natural range 0 .. 2**16 - 1;
function Sum (N : Domain) return Natural is
(if N = 0 then 0 else N + Sum (N - 1))
with
Post => Sum'Result = To_Int (To_Big (N) * (1 + To_Big (N)) / 2),
Subprogram_Variant => (Decreases => N);
-- Request a proof that Sum will terminate for all possible values of N.
pragma Annotate (GNATprove, Terminating, Sum);
begin
null;
end Main;
output (gnatprove)
$ gnatprove -Pdefault.gpr --output=oneline --report=all --level=1 --prover=z3
Phase 1 of 2: generation of Global contracts ...
Phase 2 of 2: flow analysis and proof ...
main.adb:13:13: info: subprogram "Sum" will terminate, terminating annotation has been proved
main.adb:14:30: info: overflow check proved
main.adb:14:32: info: subprogram variant proved
main.adb:14:39: info: range check proved
main.adb:16:18: info: postcondition proved
main.adb:16:31: info: range check proved
main.adb:16:53: info: predicate check proved
main.adb:16:69: info: division check proved
main.adb:16:71: info: predicate check proved
Summary logged in [...]/gnatprove.out
ADDENDUM (in response to comment)
So you can add the post condition as a recursive function, but that does not help in proving the absence of overflow; you will still have to provide some upper bound on the function result in order to convince the prover that the expression N + Sum (N - 1) will not cause an overflow.
To check the absence of overflow during the addition, the prover will consider all possible values that Sum might return according to it's specification and see if at least one of those value might cause the addition to overflow. In the absence of an explicit bound in the post condition, Sum might, according to its return type, return any value in the range Natural'Range. That range includes Natural'Last and that value will definitely cause an overflow. Therefore, the prover will report that the addition might overflow. The fact that Sum never returns that value given its allowable input values is irrelevant here (that's why it reports might). Hence, a more precise upper bound on the return value is required.
If an exact upper bound is not available, then you'll typically fallback onto a more conservative bound like, in this case, N * N (or use saturation math as shown in the Fibonacci example from the SPARK user manual, section 5.2.7, but that approach does change your function which might not be desirable).
Here's an alternative example:
example.ads
package Example with SPARK_Mode is
subtype Domain is Natural range 0 .. 2**15;
function Sum (N : Domain) return Natural
with Post =>
Sum'Result = (if N = 0 then 0 else N + Sum (N - 1)) and
Sum'Result <= N * N; -- conservative upper bound if the closed form
-- solution to the recursive function would
-- not exist.
end Example;
example.adb
package body Example with SPARK_Mode is
function Sum (N : Domain) return Natural is
begin
if N = 0 then
return N;
else
return N + Sum (N - 1);
end if;
end Sum;
end Example;
output (gnatprove)
$ gnatprove -Pdefault.gpr --output=oneline --report=all
Phase 1 of 2: generation of Global contracts ...
Phase 2 of 2: flow analysis and proof ...
example.adb:8:19: info: overflow check proved
example.adb:8:28: info: range check proved
example.ads:7:08: info: postcondition proved
example.ads:7:45: info: overflow check proved
example.ads:7:54: info: range check proved
Summary logged in [...]/gnatprove.out
subtype Summable is Natural range 0 .. 2;might help. Also,Post => Sum'Result = N * (N + 1) / 2will probably work. Finally, as N is an in parameter, it cannot be decreased, but I'm not clear what that is supposed to mean in this context. – OckhamDecreases => n, it says that any recursive invocation is done using a smaller inputnfor each invocation. This is necessary for ensuring the recursion is finite, however it would not seem to apply on recursive expressions inside postconditions, which is unfortunate in my scenario. – Conundrum