Branch stack effects
Factor handbook » The language » Stack effect checking

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Conditionals such as if and combinators built on top have the same restrictions as inline combinators (see Combinator stack effects ) with the additional requirement that all branches leave the stack at the same height. If this is not the case, the stack checker throws an unbalanced-branches-error.

If all branches leave the stack at the same height, then the stack effect of the conditional is just the maximum of the stack effect of each branch. For example,
[ [ + ] [ drop ] if ] infer.
( x x x -- x )

The call to if takes one value from the stack, a generalized boolean. The first branch [ + ] has stack effect ( x x -- x ) and the second has stack effect ( x -- ). Since both branches decrease the height of the stack by one, we say that the stack effect of the two branches is ( x x -- x ), and together with the boolean popped off the stack by if, this gives a total stack effect of ( x x x -- x ).