Type annotations

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: 

We get an ERROR: TypeError: in typeassert, expected Float64, got Int64 error message.

This is, in addition to the method specialization ...

Get Julia 1.0 Programming now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.