required keyword arguments (#586)

* required keyword arguments working

* using @test_throws and UndeKeywordError now

* defining UndefKeywordError only if not yet in Base

* corrected UndefKeywordError message

* added README entry for required keyword arguments

* Update README.md

* updated README with link

README.md

@@ -79,6 +79,8 @@ Currently, the `@compat` macro supports the following syntaxes:
   `foo(::CartesianRange{CartesianIndex{N}})` ([#20974]). Note that
   `CartesianRange` now has two type parameters, so using them as
   fields in other `struct`s requires manual intervention.
+
+* Required keyword arguments ([#25830]). For example, `@compat foo(; x, y)` makes `x` and `y` required keyword arguments: when calling `foo`, an error is thrown if `x` or `y` is not explicitly provided.
 
 ## Module Aliases
 
@@ -633,6 +635,7 @@ includes this fix. Find the minimum version from there.
 [#25780]: https://github.com/JuliaLang/julia/issues/25780
 [#25812]: https://github.com/JuliaLang/julia/issues/25812
 [#25819]: https://github.com/JuliaLang/julia/issues/25819
+[#25830]: https://github.com/JuliaLang/julia/issues/25830
 [#25873]: https://github.com/JuliaLang/julia/issues/25873
 [#25896]: https://github.com/JuliaLang/julia/issues/25896
 [#25935]: https://github.com/JuliaLang/julia/issues/25935

src/compatmacro.jl

@@ -25,13 +25,31 @@ else
     new_style_typealias(ex) = false
 end
 
+if !isdefined(Base, :UndefKeywordError)
+    struct UndefKeywordError <: Exception
+        kw
+    end
+    Base.showerror(io::IO, e::UndefKeywordError) = print(io, "UndefKeywordError: keyword argument $(e.kw) not assigned")
+    export UndefKeywordError
+end
+
+"Convert a functions symbol argument to the corresponding required keyword argument."
+function symbol2kw(sym::Symbol)
+    Expr(:kw, sym, Expr(:call, throw, UndefKeywordError(sym)))
+end
+symbol2kw(arg) = arg
+
 function _compat(ex::Expr)
     if ex.head === :call
         f = ex.args[1]
         if VERSION < v"0.6.0-dev.826" && length(ex.args) == 3 && # julia#18510
                 istopsymbol(withincurly(ex.args[1]), :Base, :Nullable)
             ex = Expr(:call, f, ex.args[2], Expr(:call, :(Compat._Nullable_field2), ex.args[3]))
         end
+        if !isdefined(Base, :UndefKeywordError) && length(ex.args) > 1 && isexpr(ex.args[2], :parameters)
+            params = ex.args[2]
+            params.args = map(symbol2kw, params.args)
+        end
     elseif ex.head === :curly
         f = ex.args[1]
         if VERSION < v"0.6.0-dev.2575" #20414

test/runtests.jl

@@ -1898,6 +1898,24 @@ let
     @test Compat.split(str, r"\.+:\.+"; limit=3, keepempty=true) == ["a","ba","cba.:.:.dcba.:."]
 end
 
+let
+    # test required keyword arguments
+    @compat func1() = 1
+    @test func1() == 1 # using the function works
+    @compat func2(x) = x
+    @test func2(3) == 3 # using the function works
+    @compat func3(;y) = y
+    @test func3(y=2) == 2 # using the function works
+    @test_throws UndefKeywordError func3()
+    @compat func4(x; z) = x*z
+    @test func4(2,z=3) == 6 # using the function works
+    @test_throws UndefKeywordError func4(2)
+    @compat func5(;x=1, y) = x*y
+    @test func5(y=3) == 3
+    @test func5(y=3, x=2) == 6
+    @test_throws UndefKeywordError func5(x=2)
+end
+
 # 0.7.0-beta.73
 let a = rand(5,5)
     s = mapslices(sort, a, dims=[1])