Rely less on inventing values to determine eltype. Fixes #7.

Author: timholy

src/MappedArrays.jl

@@ -25,7 +25,7 @@ end
 creates a view of the array `A` that applies `f` to every element of
 `A`. The view is read-only (you can get values but not set them).
 """
-mappedarray{T,N}(f, data::AbstractArray{T,N}) = ReadonlyMappedArray{typeof(f(one(T))),N,typeof(data),typeof(f)}(f, data)
+mappedarray{T,N}(f, data::AbstractArray{T,N}) = ReadonlyMappedArray{typeof(f(testvalue(data))),N,typeof(data),typeof(f)}(f, data)
 
 """
     mappedarray((f, finv), A)
@@ -36,7 +36,7 @@ the view and, correspondingly, the values in `A`.
 """
 function mappedarray{T,N}(f_finv::Tuple{Any,Any}, data::AbstractArray{T,N})
     f, finv = f_finv
-    MappedArray{typeof(f(one(T))),N,typeof(data),typeof(f),typeof(finv)}(f, finv, data)
+    MappedArray{typeof(f(testvalue(data))),N,typeof(data),typeof(f),typeof(finv)}(f, finv, data)
 end
 
 """
@@ -60,4 +60,11 @@ parenttype{T,N,A,F,Finv}(::Type{MappedArray{T,N,A,F,Finv}}) = A
 @propagate_inbounds Base.setindex!{T}(A::MappedArray{T}, val::T, i::Int...) = A.data[i...] = A.finv(val)
 @inline Base.setindex!{T}(A::MappedArray{T}, val, i::Int...) = setindex!(A, convert(T, val), i...)
 
+function testvalue(data)
+    if !isempty(data)
+        return first(data)
+    end
+    zero(eltype(data))
+end
+
 end # module

test/runtests.jl

@@ -65,3 +65,10 @@ aabs = mappedarray(abs, a)
 for i = -2:2
     @test aabs[i] == abs(a[i])
 end
+
+# issue #7
+astr = mappedarray(length, ["abc", "onetwothree"])
+@test eltype(astr) == Int
+@test astr == [3, 11]
+a = mappedarray(x->x+0.5, Int[])
+@test eltype(a) == Float64