handle empty collections with tmapreduce and tforeach

CHANGELOG.md

@@ -18,6 +18,7 @@ Version 0.5.0
 - ![BREAKING][badge-breaking] The (already deprecated) `SpawnAllScheduler` has been dropped.
 - ![BREAKING][badge-breaking] The default value for `ntasks`/`nchunks` for `DynamicScheduler` has been changed from `2*nthreads()` to `nthreads()`. With the new value we now align with `@threads :dynamic`. The old value wasn't giving good load balancing anyways and choosing a higher value penalizes uniform use cases even more. To get the old behavior, set `nchunks=2*nthreads()`.
 - ![Bugfix][badge-bugfix] When using the `GreedyScheduler` in combination with `tmapreduce` (or functions that build upon it) there could be non-deterministic errors in some cases (small input collection, not much work per element, see [#82](https://github.com/JuliaFolds2/OhMyThreads.jl/issues/82)). These cases should be fixed now.
+- ![Bugfix][badge-bugfix] We now handle empty collections as input in `tmapreduce` and `tforeach` explicitly ([#86](https://github.com/JuliaFolds2/OhMyThreads.jl/issues/86)). Our general philosophy is to try match the behavior of the serial `Base` functions.
 
 Version 0.4.6
 -------------

src/implementation.jl

@@ -60,6 +60,10 @@ function tmapreduce(f, op, Arrs...;
         kwargs...)
     mapreduce_kwargs = isgiven(init) ? (; init) : (;)
     _scheduler = _scheduler_from_userinput(scheduler; kwargs...)
+    if isempty(first(Arrs))
+        isempty(mapreduce_kwargs) && reduce_empty_err()
+        return mapreduce_kwargs.init
+    end
 
     # @show _scheduler
     if _scheduler isa SerialScheduler
@@ -74,6 +78,10 @@ end
     throw(ArgumentError("Providing an explicit scheduler as well as direct keyword arguments (e.g. $(kwargstr)) is currently not supported."))
 end
 
+@noinline function reduce_empty_err()
+    throw(ArgumentError("reducing over an empty collection is not allowed; consider supplying `init`"))
+end
+
 treducemap(op, f, A...; kwargs...) = tmapreduce(f, op, A...; kwargs...)
 
 # DynamicScheduler: AbstractArray/Generic
@@ -174,7 +182,7 @@ function _tmapreduce(f,
 end
 
 # NOTE: once v1.12 releases we should switch this to wait(t; throw=false)
-wait_nothrow(t) = Base._wait(t);
+wait_nothrow(t) = Base._wait(t)
 
 # GreedyScheduler
 function _tmapreduce(f,

test/runtests.jl

@@ -341,4 +341,18 @@ end;
     end
 end;
 
+@testset "empty collections" begin
+    for empty_coll in (11:9, Float64[])
+        for f in (sin, x->im*x, identity)
+            for op in (+, *, min)
+                @test_throws ArgumentError tmapreduce(f, op, empty_coll)
+                for init in (0.0, 0, 0.0*im, 0f0)
+                    @test tmapreduce(f, op, empty_coll; init) == init
+                end
+                @test tforeach(f, empty_coll) |> isnothing
+            end
+        end
+    end
+end;
+
 # Todo way more testing, and easier tests to deal with