foldr fails for unicode strings

[stevengj]

As reported on discourse:

julia> foldr((x, y) -> (x * '|' * y), "λ x.x")
ERROR: StringIndexError("λ x.x", 2)
Stacktrace:
 [1] string_index_err(::String, ::Int64) at ./strings/string.jl:12
 [2] getindex_continued(::String, ::Int64, ::UInt32) at ./strings/string.jl:218
 [3] getindex at ./strings/string.jl:211 [inlined]
 [4] mapfoldr_impl(::typeof(identity), ::getfield(Main, Symbol("##5#6")), ::NamedTuple{(:init,),Tuple{Char}}, ::String, ::Int64) at ./reduce.jl:104
 [5] mapfoldr_impl(::Function, ::Function, ::NamedTuple{(),Tuple{}}, ::String, ::Int64) at ./reduce.jl:115
 [6] #mapfoldr#189 at ./reduce.jl:125 [inlined]
 [7] mapfoldr at ./reduce.jl:125 [inlined]
 [8] #foldr#190 at ./reduce.jl:144 [inlined]
 [9] foldr(::Function, ::String) at ./reduce.jl:144
 [10] top-level scope at none:0

I would consider this a bug, not just a limitation of foldr, because it silently works fine for ASCII strings.

[stevengj]

The issue here is that the mapfoldr implementation does reverse iteration by itr[i - 1], assuming the iterator is indexable and that the indices are consecutive. The right thing is probably to use Iterators.reverse (#24823).

[stevengj]

For example, myfoldr(f, x) = foldl((x,y) -> f(y,x), Iterators.reverse(x)) works.

[stevengj]

I should have a PR shortly.

[stevengj]

Actually there is already a PR #25520 that should do the trick.