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Type ascriptions

A type ascription is a way of telling the type checker the expected type of some expression.

def foo = 1 : f64

It is used to disambiguate otherwise ambiguous cases. For example, the literal 1 above would be inferred to have type i32 if not for the type ascription.

For return types, we put the ascription on the definition itself:

def bar : f64 = 1

This is equivalent to the above, but more common.

Most Futhark programs do not need explicit type ascriptions (although it is good form to list return types), but it can be needed to disambiguate some tricky cases. For example, if we want to create an empty two-dimensional array with a specific inner size:

def baz = [] : [0][10]f32

Type ascriptions are not “casts” and cannot change the type of a value.

See also

Converting a value to a different type, Shape coercions.