Merge pull request #29 from shashi/nullable

Add Nullable{T}

README.md

@@ -68,6 +68,9 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `Dict(ks, vs)` is now `Dict(zip(ks, vs))` [#8521](https://github.com/JuliaLang/julia/pull/8521)
 
+## Nullable type
+* [`Nullable` types](http://julia.readthedocs.org/en/latest/manual/types/?highlight=nullable#nullable-types-representing-missing-values) and their associated operations.
+
 ## Developer tips
 
 If you're adding additional compatibility code to this package, the following shell script is useful for extracting the version number from a git commit SHA:

src/Compat.jl

@@ -173,4 +173,8 @@ end
 
 export @compat, @inline, @noinline
 
+if VERSION < v"0.4.0-dev+656"
+    include("nullable.jl")
+end
+
 end # module

src/nullable.jl

@@ -0,0 +1,63 @@
+import Base: eltype, convert, get, isequal, ==, hash, show
+export Nullable, NullException, isnull
+
+immutable Nullable{T}
+    isnull::Bool
+    value::T
+
+    Nullable() = new(true)
+    Nullable(value::T) = new(false, value)
+end
+
+immutable NullException <: Exception
+end
+
+Nullable{T}(value::T) = Nullable{T}(value)
+Nullable() = Nullable{Union()}()
+
+eltype{T}(::Type{Nullable{T}}) = T
+
+function convert{T}(::Type{Nullable{T}}, x::Nullable)
+    return isnull(x) ? Nullable{T}() : Nullable{T}(convert(T, get(x)))
+end
+
+convert{T}(::Type{Nullable{T}}, x::T) = Nullable{T}(x)
+
+convert{T}(::Type{Nullable{T}}, ::Void) = Nullable{T}()
+convert(   ::Type{Nullable   }, ::Void) = Nullable{Union()}()
+
+function show{T}(io::IO, x::Nullable{T})
+    if x.isnull
+        @printf(io, "Nullable{%s}()", repr(T))
+    else
+        @printf(io, "Nullable(%s)", repr(x.value))
+    end
+end
+
+get(x::Nullable) = x.isnull ? throw(NullException()) : x.value
+
+get{T}(x::Nullable{T}, y) = x.isnull ? convert(T, y) : x.value
+
+isnull(x::Nullable) = x.isnull
+
+function isequal(x::Nullable, y::Nullable)
+    if x.isnull && y.isnull
+        return true
+    elseif x.isnull || y.isnull
+        return false
+    else
+        return isequal(x.value, y.value)
+    end
+end
+
+==(x::Nullable, y::Nullable) = throw(NullException())
+
+const nullablehash_seed = UInt === UInt64 ? 0x932e0143e51d0171 : 0xe51d0171
+
+function hash(x::Nullable, h::UInt)
+    if x.isnull
+        return h + nullablehash_seed
+    else
+        return hash(x.value, h + nullablehash_seed)
+    end
+end