The following example can be tested on https://ellie-app.com/m9tk39VpQg/0.
import Html exposing (..)
import Json.Decode
payload =
"""
["fu", "bar"]
"""
main =
Json.Decode.decodeString decoder payload -- Ok ["fu","bar"]
|> toString
|> text
decoder =
Json.Decode.list Json.Decode.string