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# Size parameters

Size annotations are used for expressing constraints on the size of array-typed function parameters, and guarantees on the result. For example, we can promise that a function always returns a single-element array:

For expressing constraints among the sizes of the parameters, Futhark provides *size parameters*. Mathematically, the dot product is only defined for vectors of the same length. Size parameters let us express this in Futhark.

As type parameters, size parameters are not passed explicitly when calling the function, but inferred from the value arguments.