Merge pull request #132 from serenity4/tryswitch

Implement @tryswitch

src/MatchImpl.jl

@@ -1,6 +1,6 @@
 module MatchImpl
 export is_enum,
-    pattern_uncall, pattern_unref, pattern_unmacrocall, @switch, @match, Where, gen_match, gen_switch
+    pattern_uncall, pattern_unref, pattern_unmacrocall, @switch, @tryswitch, @match, Where, gen_match, gen_switch
 export Q
 import MLStyle
 using MLStyle: mlstyle_report_deprecation_msg!
@@ -384,6 +384,36 @@ macro switch(val, ex)
     res = init_cfg(res)
     esc(res)
 end
+"""
+    @tryswitch <item> begin
+        @case <pattern>
+            <action>
+    end
+
+Very similar to [`@switch`](@ref), except that a failure to match does nothing instead of throwing a "match non-exhaustive" error.
+
+It is equivalent to
+
+```julia
+@switch <item> begin
+    @case <pattern>
+        <action>
+    @case _
+        nothing
+    end
+```
+"""
+macro tryswitch(val, ex)
+    @assert Meta.isexpr(ex, :block)
+    insert_case = length(ex.args) >= 2 || begin
+        case, line = ex.args[end-2:end]
+        Meta.isexpr(case, [:macrocall], 3)  &&
+            case.args[1] == Symbol("@case") &&
+            case.args[3] == :_
+    end
+    insert_case && push!(ex.args, Expr(:macrocall, Symbol("@case"), __source__, :_), :nothing)
+    :($(esc(:($(@__MODULE__).@switch $val $ex))))
+end
 @specialize
 function gen_switch(val, ex, __source__::LineNumberNode, __module__::Module)
     @assert Meta.isexpr(ex, :block)

test/runtests.jl

@@ -54,6 +54,7 @@ MODULE = TestModule
 
 include("issues/109.jl")
 include("when.jl")
+include("switch.jl")
 include("untyped_lam.jl")
 include("active_patterns.jl")
 include("uncomp.jl")

test/switch.jl

@@ -0,0 +1,49 @@
+@testcase "@switch" begin
+    flag = Ref(0)
+    function try_setflag(x)
+        @switch x begin
+            @case (::Integer, 2)
+                flag[] = 1
+            @case ::Bool
+                nothing
+                flag[] = 2
+            @case ::String
+                flag[] = 0
+        end
+    end
+
+    try_setflag((1, 2))
+    @test flag[] == 1
+    try_setflag(false)
+    @test flag[] == 2
+    try_setflag("")
+    @test flag[] == 0
+    # Non-exhaustive matches throw an error.
+    @test_throws ErrorException try_setflag(:a)
+
+    flag2 = Ref(0)
+    function try_setflag_2(x)
+        @tryswitch x begin
+            @case (::Integer, 2)
+                flag[] = 1
+            @case ::Bool
+                flag[] = 2
+                @tryswitch x begin
+                    @case true
+                        flag2[] = 2
+                end
+        end
+    end
+
+    try_setflag_2((1, 2))
+    @test flag[] == 1
+    try_setflag_2(false)
+    @test flag[] == 2
+    @test flag2[] == 0
+    try_setflag_2(true)
+    @test flag[] == 2
+    @test flag2[] == 2
+
+    # Non-exhaustive matches fail silently.
+    try_setflag_2("")
+end