An interface is a shared way to use different implementations. A mouse and keyboard let you operate different computers without knowing their hardware; likewise, code can use arrays and ranges without caring how each stores its elements. Both support indexing and iteration, even though a range’s elements are implicit. section 12.1
One function, two array implementations
In Julia, `function addup(xs::AbstractArray); total = 0; for x in xs; total += x; end; total; end` returns `12` for both `addup(3:5)` and `addup([3, 4, 5])`. It rejects `addup((3, 4, 5))`: a tuple is not an `AbstractArray`. That restriction belongs to `addup`, not to addition in general—`sum((3, 4, 5))` works. section 12.1
Julia interfaces are informal: there is no special interface declaration. Define functions and document which methods another type must provide. Methods belong to functions rather than types, so an abstract type alone does not tell an implementer every required operation. section 12.2
Make a tank contract discoverable
Suppose `refill!(tank::Tank)` calls `totalmass(tank)` and `drymass(tank)`. A new `Tank` subtype can have fields with those names yet still lack the required methods. With no `totalmass` function defined, refilling produces `UndefVarError`; defining the documented generic function changes the missing-implementation failure to a `MethodError`. Documenting `totalmass(t::Tank) -> Float64` tells implementers what to supply. section 12.2
An interface can also supply some behavior. For a `Cipher`, character-level `encrypt` and `decrypt` methods can be required while string-level methods map those operations over characters. Explicit fallback errors can tell developers what to implement. A password service holding an abstract `Cipher` can then use either a Caesar cipher or a substitution cipher without changing its own encryption calls. section 15.6
Listing 15.10 Defining an abstract cipher interface
abstract type Cipher end
function encrypt(cipher::Cipher, char::Char)
error("Implement encrypt(::", typeof(cipher), ", char)")
end
function decrypt(cipher::Cipher, char::Char)
error("Implement decrypt(::", typeof(cipher), ", char)")
end
function encrypt(cipher::Cipher, message::AbstractString)
map(ch -> encrypt(cipher, ch), message)
end
function decrypt(cipher::Cipher, ciphertext::AbstractString)
map(ch -> decrypt(cipher, ch), ciphertext)
end
The cipher listing shows the distinction between character methods that signal a missing implementation and string methods that reuse them through `map`. section 15.6
The same payoff applies to collections: make a custom structure such as a staged rocket iterable, and functions including `map`, `reduce`, `filter`, `sum`, and `collect` become available to it. section 12.0 section 12.7
