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Holes

A hole is a placeholder expression we can use when we don’t yet know what to put:

def foo [n] (x: [n]f32) : i64 = ???

The type checker will issue a warning for each hole encountered, listing the inferred type:

Warning at holes.fut:6:28-30:
  Hole of type: i64

Holes can have any type, including functions:

def bar [n] (x: [n]f32) : i64 = ??? x

Which will warn us:

holes.fut:18:33-35:
  Hole of type: [n]f32 -> i64

Hole expressions make it possible to sketch out the design of a program by defining only the types of top-level functions, and then gradually fill in the blanks. We can also compile and run a program with holes in it, but executing a hole results in a run-time error.

Note that because holes are subject to ordinary type inference, they must have an unambiguous type. For example, the expression length ??? will fail to type-check because the type checker has no way to determine the element type or size of the placeholder array. You can use type ascriptions to disambiguate such cases, if necessary:

def baz n = length (??? : [n]i32)

See also

Type ascriptions