Haskell使用forEach渲染HTML节点时遇类型匹配错误

错误信息 #

Web/View/HardwareVendors/Show.hs:7:33
    • Couldn't match expected type ‘Text’
                  with actual type ‘Data.MonoTraversable.Element mono0’
      The type variable ‘mono0’ is ambiguous
    • In the second argument of ‘forEach’, namely ‘renderInput’
      In the first argument of ‘toHtml’, namely
        ‘((forEach hardwareVendor.required_fields) renderInput)’
      In the expression:
        toHtml ((forEach hardwareVendor.required_fields) renderInput)
  |
7 |     html ShowView { .. } = [hsx|
  |                                 ^...

相关源码 #

module Web.View.HardwareVendors.Show where
import Web.View.Prelude

data ShowView = ShowView { hardwareVendor :: HardwareVendor }

instance View ShowView where
    html ShowView { .. } = [hsx|
        {breadcrumb}
        <h1>{hardwareVendor.name}</h1>
        <form action="{hardwareVendor.getUrl}" method="get">
            -- required_fields is a Text[] postgres column
            {forEach hardwareVendor.required_fields renderInput}
            <input type="submit" value="Submit">
        </form>
    |]
        where
            breadcrumb = renderBreadcrumb
                            [ breadcrumbLink "HardwareVendors" HardwareVendorsAction
                            , breadcrumbText "Show HardwareVendor"
                            ]
renderInput :: Text -> Html
renderInput field = [hsx|
    <label for="{field}">{field}</label>
    <input type="text" id="{field}" name="{field}" required>
    <br>
|]

其中required_fields是Postgres的Text[]类型列，renderInput接受Text参数并返回Html，使用forEach遍历该字段时触发上述类型错误。

解决方法 #

问题根源是IHP中PostgreSQL的数组类型会映射为Array Text（来自Database.PostgreSQL.Simple.Array），而非Haskell原生的[Text]列表，导致forEach无法自动推断元素类型。

方法1：转换为原生列表 #

导入Database.PostgreSQL.Simple.Array模块（如果尚未导入），然后用toList将Array Text转为普通列表：

{forEach (toList hardwareVendor.required_fields) renderInput}

方法2：显式指定类型 #

给forEach添加类型注解，明确元素类型为Text：

{forEach @Text hardwareVendor.required_fields renderInput}

内容的提问来源于stack exchange，提问作者Geoff Langenderfer