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| -rw-r--r-- | julia.html.markdown | 151 |
1 files changed, 131 insertions, 20 deletions
diff --git a/julia.html.markdown b/julia.html.markdown index f8c2f6e7..85aca49d 100644 --- a/julia.html.markdown +++ b/julia.html.markdown @@ -479,49 +479,104 @@ filter(x -> x > 5, [3, 4, 5, 6, 7]) #=> [6, 7] [add_10(i) for i in [1, 2, 3]] #=> [11, 12, 13] #################################################### -## 5. Types and Multiple-Dispatch +## 5. Types #################################################### -# Type definition +# Julia has a type system. +# Every value has a type; variables do not have types themselves. +# You can use the `typeof` function to get the type of a value. +typeof(5) #=> Int64 + +# Types are first-class values +typeof(Int64) #=> DataType +typeof(DataType) #=> DataType +# DataType is the type that represents types, including itself. + +# Types are used for documentation, optimizations, and dispatch. +# They are not statically checked. + +# Users can define types +# They are like records or structs in other languages. +# New types are defined used the `type` keyword. + +# type Name +# field::OptionalType +# ... +# end type Tiger taillength::Float64 - coatcolor # no type annotation is implicitly Any + coatcolor # not including a type annotation is the same as `::Any` end -# default constructor is the properties in order -# so, Tiger(taillength,coatcolor) -# Type instantiation -tigger = Tiger(3.5,"orange") # the type doubles as the constructor function +# The default constructor's arguments are the properties +# of the tyep, in order the order they are listed in the definition +tigger = Tiger(3.5,"orange") #=> Tiger(3.5,"orange") + +# The type doubles as the constructor function for values of that type +sherekhan = typeof(tigger)(5.6,"fire") #=> Tiger(5.6,"fire") -# Abtract Types -abstract Cat # just a name and point in the type hierarchy +# These struct-style types are called concrete types +# They can be instantiated, but cannot have subtypes. +# The other kind of types is abstract types. -# * types defined with the type keyword are concrete types; they can be -# instantiated -# -# * types defined with the abstract keyword are abstract types; they can -# have subtypes. -# -# * each type has one supertype; a supertype can have zero or more subtypes. +# abstract Name +abstract Cat # just a name and point in the type hierarchy +# Abstract types cannot be instantiated, but can have subtypes. +# For example, Number is an abstract type +subtypes(Number) #=> 6-element Array{Any,1}: + # Complex{Float16} + # Complex{Float32} + # Complex{Float64} + # Complex{T<:Real} + # ImaginaryUnit + # Real +subtypes(Cat) #=> 0-element Array{Any,1} + +# Every type has a super type; use the `super` function to get it. +typeof(5) #=> Int64 +super(Int64) #=> Signed +super(Signed) #=> Real +super(Real) #=> Number +super(Number) #=> Any +super(super(Signed)) #=> Number +super(Any) #=> Any +# All of these type, except for Int64, are abstract. + +# <: is the subtyping operator type Lion <: Cat # Lion is a subtype of Cat mane_color roar::String end +# You can define more constructors for your type +# Just define a function of the same name as the type +# and call an existing constructor to get a value of the correct type +Lion(roar::String) = Lion("green",roar) +# This is an outer constructor because it's outside the type definition + type Panther <: Cat # Panther is also a subtype of Cat eye_color Panther() = new("green") # Panthers will only have this constructor, and no default constructor. end +# Using inner constructors, like Panter does, gives you control +# over how values of the type can be created. +# When possible, you should use outer constructors rather than inner ones. -# Multiple Dispatch +#################################################### +## 6. Multiple-Dispatch +#################################################### # In Julia, all named functions are generic functions # This means that they are built up from many small methods -# For example, let's make a function meow: +# Each constructor for Lion is a method of the generic function Lion. + +# For a non-constructor example, let's make a function meow: + +# Definitions for Lion, Panther, Tiger function meow(cat::Lion) - cat.roar # access properties using dot notation + cat.roar # access type properties using dot notation end function meow(cat::Panther) @@ -532,21 +587,76 @@ function meow(cat::Tiger) "rawwwr" end +# Testing the meow function meow(tigger) #=> "rawwr" meow(Lion("brown","ROAAR")) #=> "ROAAR" meow(Panther()) #=> "grrr" +# Review the local type hierarchy +issubtype(Tiger,Cat) #=> false +issubtype(Lion,Cat) #=> true +issubtype(Panther,Cat) #=> true + +# Defining a function that takes Cats function pet_cat(cat::Cat) println("The cat says $(meow(cat))") end +pet_cat(Lion("42")) #=> prints "The cat says 42" try pet_cat(tigger) #=> ERROR: no method pet_cat(Tiger,) catch e println(e) end -pet_cat(Lion(Panther(),"42")) #=> prints "The cat says 42" +# In OO languages, single dispatch is common; +# this means that the method is picked based on the type of the first argument. +# In Julia, all of the argument types contribute to selecting the best method. + +# Let's define a function with more arguments, so we can see the difference +function fight(t::Tiger,c::Cat) + println("The $(t.coatcolor) tiger wins!") +end +#=> fight (generic function with 1 method) + +fight(tigger,Panther()) #=> prints The orange tiger wins! +fight(tigger,Lion("ROAR")) #=> prints The orange tiger wins! + +# Let's change the behavior when the Cat is specifically a Lion +fight(t::Tiger,l::Lion) = println("The $(l.mane_color)-maned lion wins!") +#=> fight (generic function with 2 methods) + +fight(tigger,Panther()) #=> prints The orange tiger wins! +fight(tigger,Lion("ROAR")) #=> prints The green-maned lion wins! + +# We don't need a Tiger in order to fight +fight(l::Lion,c::Cat) = println("The victorious cat says $(meow(c))") +end +#=> fight (generic function with 3 methods) + +fight(Lion("balooga!"),Panther()) #=> prints The victorious cat says grrr +try + fight(Panther(),Lion("RAWR")) #=> ERROR: no method fight(Panther,Lion) +catch +end + +# Also let the cat go first +fight(c::Cat,l::Lion) = println("The cat beats the Lion") +#=> Warning: New definition +# fight(Cat,Lion) at none:1 +# is ambiguous with +# fight(Lion,Cat) at none:2. +# Make sure +# fight(Lion,Lion) +# is defined first. +#fight (generic function with 4 methods) + +# This warning is because it's unclear which fight will be called in: +fight(Lion("RAR"),Lion("brown","rarrr")) #=> prints The victorious cat says rarrr +# The result may be different in other versions of Julia + +fight(l::Lion,l2::Lion) = println("The lions come to a tie") +fight(Lion("RAR"),Lion("brown","rarrr")) #=> prints The lions come to a tie ``` @@ -554,3 +664,4 @@ pet_cat(Lion(Panther(),"42")) #=> prints "The cat says 42" You can get a lot more detail from [The Julia Manual](http://docs.julialang.org/en/latest/manual/) +The best place to get help with Julia is the (very friendly) [mailing list](https://groups.google.com/forum/#!forum/julia-users). |