Add generic isnull, unsafe_get methods

Also move isnull docstring out of helpdb and into base/nullable.jl.

Author: davidagold

base/docs/helpdb/Base.jl

@@ -1466,13 +1466,6 @@ julia> log2(10)
 """
 log2
 
-"""
-    isnull(x)
-
-Is the `Nullable` object `x` null, i.e. missing a value?
-"""
-isnull
-
 """
     abs2(x)
 

base/exports.jl

@@ -1324,6 +1324,7 @@ export
 
 # nullable types
     isnull,
+    unsafe_get,
 
 # Macros
     # parser internal

base/nullable.jl

@@ -61,7 +61,25 @@ end
 
 get(x::Nullable) = x.isnull ? throw(NullException()) : x.value
 
+"""
+    unsafe_get(x)
+
+Return the value of `x` for `x::Nullable`; return `x` for all other `x`.
+
+Unsafe because does not check whether or not `x` is null before attempting to
+access value of `x` for `x::Nullable`.
+"""
+unsafe_get(x::Nullable) = x.value
+unsafe_get(x) = x
+
+"""
+    isnull(x)
+
+Return whether or not `x` is null for `x::Nullable`; return `false` for all
+other `x`.
+"""
 isnull(x::Nullable) = x.isnull
+isnull(x) = false
 
 
 ## Operators

test/nullable.jl

@@ -161,15 +161,51 @@ for T in types
     @test get(x3, zero(T)) === one(T)
 end
 
-# isnull(x::Nullable)
 for T in types
+    # unsafe_get(x::Nullable)
+    x1 = Nullable{T}()
+    x2 = Nullable(zero(T))
+    x3 = Nullable(one(T))
+    a = rand(T)
+    x4 = Nullable(a)
+
+    @test isa(unsafe_get(x1), T)
+    @test unsafe_get(x2) === zero(T)
+    @test unsafe_get(x3) === one(T)
+    @test unsafe_get(x4) === a
+
+    # unsafe_get(x)
+    x2 = zero(T)
+    x3 = one(T)
+    x4 = rand(T)
+
+    @test unsafe_get(x2) === zero(T)
+    @test unsafe_get(x3) === one(T)
+    @test unsafe_get(x4) === x4
+end
+
+@test_throws UndefRefError unsafe_get(Nullable())
+@test_throws UndefRefError unsafe_get(Nullable{String}())
+@test_throws UndefRefError unsafe_get(Nullable{Array}())
+
+for T in types
+    # isnull(x::Nullable)
     x1 = Nullable{T}()
     x2 = Nullable(zero(T))
     x3 = Nullable(one(T))
 
     @test isnull(x1) === true
     @test isnull(x2) === false
     @test isnull(x3) === false
+
+    # isnull(x)
+    x1 = zero(T)
+    x2 = one(T)
+    x3 = rand(T)
+
+    @test isnull(x1) === false
+    @test isnull(x2) === false
+    @test isnull(x3) === false
 end
 
 @test isnull(Nullable())