Principle of Least Surprise

Andrew Koenig has a post on surprises in designs:

[In APL,] a surprising result comes from three applications of the principle of least surprise:

1) It should be possible to have an array with no dimensions, and a zero-dimensional array should be the same thing as a scalar.

2) The size of an array should be a vector with one element for each dimension of the array.

3) When you do arithmetic on a scalar and a vector, the result has the same size as the vector.

Choosing the least surprising behavior in these three contexts causes the surprising behavior that the average of a scalar sum(v)/size(v) is an empty vector.