add docstring and error message

Author: jishnub

src/MappedArrays.jl

@@ -264,6 +264,22 @@ eltypes(A::AbstractArray) = Tuple{eltype(A)}
 
 # mapreduce
 
+"""
+    mappedarrayreduce(f, op, A::AbstractArray...; kw...)
+
+Perform a "lazy" `mapreduce` without allocating an intermediate array. This may 
+be more performant than a standard `mapreduce`. Functionally this is equivalent to 
+`reduce(op, mappedarray(f, A); kw...)`.
+
+# Examples
+```jldoctest
+julia> mappedarrayreduce(x -> x^2, +, 1:10) # == 1^2 + 2^2 + 3^2
+385
+```
+"""
 mappedarrayreduce(f, op, A::AbstractArray...; kw...) = reduce(op, mappedarray(f, A...); kw...)
+mappedarrayreduce(f, finv, op, A::AbstractArray...; kw...) = error(
+    "mappedarrayreduce does not support an inverse function, "*
+    "please use the signature mappedarrayreduce(f, op, A::AbstractArray...; kw...)")
 
 end # module

test/runtests.jl

@@ -188,6 +188,8 @@ end
         @test mapreduce(f, op, x, init = zero(T), dims = 1) == mappedarrayreduce(f, op, x, init = zero(T), dims = 1)
         @test mapreduce(f, op, x, init = zero(T), dims = :) == mappedarrayreduce(f, op, x, init = zero(T), dims = :)
 
+        @test_throws Exception mappedarrayreduce(x->x^2, sqrt, op, x)
+
         if VERSION >= v"1.2"
             f = ==; op = +
             @test mapreduce(f, op, x, y) == mappedarrayreduce(f, op, x, y)