As we saw in Chapter 2, Variables, Types, and Operations, type-annotating a variable is done with the ::
operator, such as in the function definition function write(io::IO, s::String) #... end
, where the parameter io
has to be of type IO
, and s
of type String
. To put it differently, io
has to be an instance of type IO
, and s
an instance of type String
. The ::
operator is, in fact, an assertion that affirms that the value on the left is of the type on the right. If this is not true, a typeassert
error is thrown. Try this out in the REPL:
# see the code in Chapter 6\conversions.jl: (31+42)::Float64
We get an ERROR: TypeError: in typeassert, expected Float64, got Int64
error message.
This is, in addition to the method specialization for multiple dispatch, an important reason why type annotations are used in function signatures.
The operator ::
can also be used in the sense of a type declaration, but only in local scope, such as in functions, as follows:
n::Int16 or local n::Int16...