如何在Haskell中从HDBC转换SqlValue？

Question

我是Haskell的初学者，在HDBC中我似乎不能从SqlValues中获取字符串。在ghci中：

*Main> import Database.HDBC
*Main Database.HDBC> toSql "test"
SqlString "test"
*Main Database.HDBC> fromSql $ toSql "test"

<interactive>:3:1: error:
    • Non type-variable argument
        in the constraint: Convertible SqlValue a
      (Use FlexibleContexts to permit this)
    • When checking the inferred type
        it :: forall a. Convertible SqlValue a => a
*Main Database.HDBC> :set -XFlexibleContexts
*Main Database.HDBC> fromSql $ toSql "test"

<interactive>:5:1: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘print’
      prevents the constraint ‘(Show a0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance [safe] Show SqlValue
          -- Defined in ‘Database.HDBC.SqlValue’
        instance [safe] Show SqlError
          -- Defined in ‘Database.HDBC.Statement’
        instance (Show b, Show a) => Show (Either a b)
          -- Defined in ‘Data.Either’
        ...plus 45 others
        ...plus 184 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In a stmt of an interactive GHCi command: print it

Answer 1

编译器无法推断出要打印的东西的类型。您可以添加类型注释(如错误消息所示)以使其工作：

Prelude Database.HDBC> fromSql $ toSql "test" :: String
"test"