Why can I not subtract 2 local variables of type nat from each other and assign that result to another local variable in a view in smartpy?

Question

I am trying to do some maths operations inside an on-chain view and am getting an unexpected result.

The reproduction contract tries to perform the following steps:

1. Define an internal contract property of type nat (e.g. 40)
2. Define a new local variable A of type nat inside the view (e.g. 1000)
3. Multiply the internal nat value by 10 and store its result in another local variable B (will be 400)
4. Subtract variable B from variable A (result is 600) and store result in another local variable C
5. Divide variable C by 2 storing it in variable D and return the value (should return 300)

import smartpy as sp

class MathsTest(sp.Contract):
    def __init__(self, **kargs):
        self.init(
            internal_number=sp.nat(40),
            **kargs
        )

    @sp.onchain_view()
    def nat_test(self):
        maxi = sp.local('maxi', sp.nat(1000))
        multi = sp.local('multi', self.data.internal_number * sp.nat(10))
        sub = sp.local('sub', maxi.value - multi.value)
        div = sp.local('div', sub.value // sp.nat(2))

        sp.result(div.value)

@sp.add_test(name = "MathsTest")
def test():
    scenario = sp.test_scenario()
    scenario.h1("Maths")
    c1 = MathsTest()
    scenario += c1

    scenario.verify(c1.nat_test() == sp.nat(300))

Instead of passing the test throws the following:

Error: Type Error
Type sp.TInt / sp.TNat mismatch
Type Error (maxi.value : sp.TNat), (multi.value : sp.TNat) cannot be subtracted in (maxi.value - multi.value : sp.TNat)
(__main__, line 14)

Why does it say there is an int mismatch when everything is declared as nat?

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Contract SmartPy Link

Answer 1

Subtractions are implicitly taken as int due to the possibility of negative results. Therefore, the variable sub is of type int here.

To resolve this, wrap sub as sp.as_nat(sub) on the dividend.