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I don't understand how they don't compose, from your example:

    (X = A + B, Y = X + C, Y = A + B + C is false)
I understand that types aren't math values, but isn't the point of using a `+` to describe the communicative value of the type so that `(A + B) + C = A + B + C`?

Also






data A = A

data B = B

data X = XA A | XB B

data Y = YA A | YB B

f :: X -> () f = undefined

let r = f (undefined :: Y)

There's absolutely no way to write this so it compiles. In fact, there isn't even a way to define the composed types so that they only express a sum, you have to add extra baggage.

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