Scotty Convert GET 参数/Lazy.Text 转换

Question

我尝试将 GET 参数传递给函数并从结果中连接一个字符串

{-# LANGUAGE OverloadedStrings #-}
module Main where
import Data.Monoid ((<>))
import Web.Scotty

f x = x <> x
main = do
  scotty 3000 $ do
    get "/f/:x" $ do
        x <- param "x"
        text ("f(" <> x <> ") = " <> f x)

为了使我的应用程序更有趣，我想使用一个需要 Num 参数类型实例的函数，例如

f x = x * x

如何将 convert/read x 转换为 Num（或 Maybe...）并将函数结果转换回 Data.Text.Internal.Lazy.Text？

我试过了

text ("f(" <> x <> ") = " <> (show $ f $ read x))

产生错误：

• Couldn't match expected type
  ‘text-1.2.3.1:Data.Text.Internal.Lazy.Text’
  with actual type ‘[Char]’

Answer 1

感谢 Bob Dalgleish（评论）帮助我解决这个问题，使用 pack/unpack 函数我可以解决这个问题

{-# LANGUAGE OverloadedStrings #-}
module Main where
import Data.Monoid ((<>))
import qualified Data.Text as T
import qualified Data.Text.Lazy as L
import Web.Scotty

f x = x * x
main = do
  scotty 3000 $ do
    get "/f/:x" $ do
        x <- param "x"
        let res_string = show $ f $ read $ T.unpack x
        let label_string = "f(" <> (T.unpack x) <> ") = "
        text $ L.pack (label_string <> res_string)

请注意 read 是 "dangerous"，不应替换为 readMaybe，但这与此处无关。

Scotty Convert GET 参数/Lazy.Text 转换

Scotty Convert GET Parameter / Lazy.Text conversion

haskell

scotty