Abstract types in Julia

Name the family, dispatch on it, override per concrete type.

# An abstract type names a family; methods dispatch on it.
abstract type Instrument end

struct Camera <: Instrument
    megapixels::Int
end
struct Spectrograph <: Instrument
    resolution::Int
end

# One generic fallback, one specific override.
setup(i::Instrument) = "generic checklist"
setup(c::Camera) = "cool sensor, $(c.megapixels) MP flat frames"

println(setup(Camera(62)))
println(setup(Spectrograph(1800)))
println(Camera <: Instrument)

How it works

1. abstract type Instrument end has no fields, only meaning.
2. Camera <: Instrument declares the subtype relation.
3. A generic method covers the family; a specific one overrides.

Keywords and builtins used here

• Camera
• Instrument
• Int
• Spectrograph
• abstract
• end
• struct
• type

The run, in numbers

Lines

17

Characters to type

439

Tokens

75

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 69 seconds.

Type this snippet

Step 2 of 3 in Types, step 17 of 27 in Language basics.

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