解码列表时忽略无效项目

Question

解码列表时是否可以忽略无效项目？\n示例：我有一个模型

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type Type\n    = A\n    | B\n\ntype alias Section =\n    { sectionType : Type\n    , index : Int\n    }\n\n\ngetTypeFromString : String -> Maybe Type\ngetTypeFromString input =\n    case input of\n        \xe2\x80\x9ca\xe2\x80\x9d ->\n            Just A\n\n        \xe2\x80\x9cb\xe2\x80\x9d ->\n            Just B\n\n        _ ->\n            Nothing\n\ndecodeType : Decoder Type\ndecodeType =\n    Decode.string\n        |> Decode.andThen\n            (\\str ->\n                case getTypeFromString str of\n                    Just sectionType ->\n                        Decode.succeed sectionType\n\n                    Nothing ->\n                        Decode.fail <| ("Unknown type" ++ str)\n            )\n\n\ndecodeSection : Decoder Section\ndecodeSection =\n    Decode.map2 Section\n        (Decode.field "type" decodeType)\n        (Decode.field "index" Decode.int)\n

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如果我解码 JSON

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{\n  "sections": [{type: "A", index: 1}, {type: "invalid-type", index: 2}]\n}\n

\n

我期望我的部分 = [ {type = A, index= 1} ]

\n

Answer 1

一般来说，处理这些问题的方法是将其解码为表示选项的 Elm 类型，然后使用map.

例如，在你的例子中，我会选择这样的东西：

decodeMaybeType : Decoder (Maybe Type)
decodeMaybeType =
    Decode.string
        |> Decode.map getTypeFromString 


decodeMaybeSection : Decoder (Maybe Section)
decodeMaybeSection =
    Decode.map2 (\maybeType index -> Maybe.map (\t -> Section t index) maybeType)
        (Decode.field "type" decodeMaybeType)
        (Decode.field "index" Decode.int)


decodeSections : Decoder (List Section)
decodeSections =
    Decode.list decodeMaybeSection
       |> Decode.map (List.filterMap identity)

注意：List.filterMap identity是 a ，它会一次性List (Maybe a) -> List a过滤掉Nothing并摆脱s 。Maybe