Lucid pass arguments to Html

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1

Hello, when the somePage does not contain any arguments, it builds. But when I try to customize this page and use a User argument, it fails on h2_ line. Why? It seems to be a so trivial thing - to print a custom text, but it fails.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ExtendedDefaultRules #-}

import Lucid

data User = User
  { name :: Text
  , email :: Text
  } deriving (Eq, Show, Generic)

somePage :: User -> Html ()
somePage (User name email) = html_ $ do
  head_ $ do
    title_ "Some Page"
  body_ $ do
    h2_ "This single line builds"
    h2_ ("But this line does not build" ++ unpack username)

The build error is:

Couldn't match type: HtmlT Data.Functor.Identity.Identity a0
                     with: [Char]
        arising from a functional dependency between:
          constraint ‘Term [Char] (HtmlT Data.Functor.Identity.Identity a0)’
            arising from a use of ‘h2_’
          instance ‘Term (HtmlT m a) (HtmlT m a)’ at <no location info>
2

Lucid’s Html type has an instance of IsString, which means that when used together with OverloadedStrings, any string literal in your program might be inferred to have type Html (). Usually this makes programs simpler, but sometimes you get subtle issues where the inferred types change with small changes of your program, as in your example:

    -- the literal here is inferred to be html
    h2_ ("This single line builds" :: Html ())
    -- but this literal must be String to use (++)
    h2_ ((("But this line does not build" :: String) ++
             unpack username) :: String)

In the first line, the literal must have type Html () to type check (but if you turn OverloadedStrings off, it won’t).

In your second line, you use (++), which operates on String — so the compiler will instead infer that this literal must have type String, and since the result of (++) also has type String, but since h2_ expects html, this is a type error.

So in this case the solution is to convert from String to Html () manually:

    h2_ ("This single line builds" :: Html ())
    h2_ (toHtml ("But this line does not build" ++ unpack username))

should type check.

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