Current, Str.fromUtf8's type signature is List U8 -> Result Str [ BadUtf8 Utf8ByteProblem Nat ]*. A possible problem with this is without reading the documentation it's unclear what Nat is supposed to be here. To clear this up, the signature could be List U8 -> Result Str [ BadUtf8 { problem : Utf8ByteProblem index : Nat } ]* to indicate that Nat is the index of the byte that failed.