The Futhark type checker is not very smart, and in some cases is very conservative in how it tracks the sizes of arrays. For example:
def A = [1,2,3] ++ [4,5,6]
As far as the type checker is concerned,
A has a completely
unknown size. Using a shape coercion we can change the size to be
what we expect:
def B = [1,2,3] ++ [4,5,6] :> i32
Shape coercions are checked dynamically, and the program will fail at run-time if the actual shape does not match the expected shape. The rank or element type of the array may not change - only the specific dimensions.