generic html forms for ADTs

this does some GHC generics witchery, now in its own module!

1 files changed, 105 insertions, 0 deletions

diff --git a/lib/Lucid/Forms.hs b/lib/Lucid/Forms.hs
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+++ b/lib/Lucid/Forms.hs
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+{-# LANGUAGE DataKinds           #-}
+{-# LANGUAGE DefaultSignatures   #-}
+{-# LANGUAGE DeriveGeneric       #-}
+{-# LANGUAGE FlexibleContexts    #-}
+{-# LANGUAGE FlexibleInstances   #-}
+{-# LANGUAGE LambdaCase          #-}
+{-# LANGUAGE OverloadedStrings   #-}
+{-# LANGUAGE RecordWildCards     #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications    #-}
+{-# LANGUAGE TypeOperators       #-}
+
+
+module Lucid.Forms (GToHtmlForm(..), GToHtmlFormInput(..), ToHtmlForm(..), ToHtmlFormInput(..)) where
+
+import           Data.Maybe           (maybeToList)
+import           Data.Proxy           (Proxy (..))
+import           Data.Text            (Text)
+import qualified Data.Text            as T
+import           Data.Time            (Day)
+import           GHC.Generics         (C1, D1, Generic (Rep), K1, Meta (..),
+                                       Rec0, S1, U1, type (:*:), type (:+:))
+import           GHC.TypeLits         (KnownSymbol, symbolVal)
+import           Lucid                (Html, ToHtml (toHtml), action_, div_,
+                                       for_, form_, id_, input_, label_,
+                                       method_, name_, placeholder_, type_,
+                                       value_)
+import           Text.ProtocolBuffers (Default (defaultValue))
+
+class GToHtmlFormInput a where
+  gtoHtmlFormInput :: Text -> Proxy (a p) -> Html ()
+
+instance (KnownSymbol name)
+  => GToHtmlFormInput (C1 (MetaCons name f s) U1) where
+  gtoHtmlFormInput group _ = do
+    input_ [type_ "radio", id_ name, value_ name, name_ group] -- need a shared name for all options here
+    label_ [for_ name] (toHtml name)
+    where name = T.pack (symbolVal (Proxy @name))
+
+instance (GToHtmlFormInput l, GToHtmlFormInput r) => GToHtmlFormInput (l :+: r) where
+  gtoHtmlFormInput group _ = do
+    gtoHtmlFormInput group (Proxy @(l _))
+    gtoHtmlFormInput group (Proxy @(r _))
+
+-- | ignore constructor & type metainfo
+instance GToHtmlFormInput a => GToHtmlFormInput (D1 r a) where
+  gtoHtmlFormInput group _ = gtoHtmlFormInput group (Proxy @(a _))
+
+-- | class for things which can be (single) options (i.e. a single input-tag or
+-- | a group of radio buttons) in an html form
+class ToHtmlFormInput a where
+  toFormInput :: Text -> Proxy a -> Html ()
+  default toFormInput :: (GToHtmlFormInput (Rep a)) => Text -> Proxy a -> Html ()
+  toFormInput group p = gtoHtmlFormInput group (Proxy @(Rep a _))
+
+instance ToHtmlFormInput Text where
+  toFormInput name _ = input_ [type_ "text", name_ name, placeholder_ name]
+instance ToHtmlFormInput Day where
+  toFormInput name _ = input_ [type_ "text", name_ name]
+
+
+-- | generic conversion of record data types to input forms
+class GToHtmlForm a where
+  gtoHtmlForm :: Proxy (a p) -> Html ()
+
+-- | each record field is a form input
+instance (KnownSymbol name, ToHtmlFormInput a)
+  => GToHtmlForm (S1 (MetaSel (Just name) su ss ds) (Rec0 a)) where
+  gtoHtmlForm _ = toFormInput (T.pack (symbolVal (Proxy @name))) (Proxy @a)
+
+-- | just chain all fields
+instance (GToHtmlForm l, GToHtmlForm r) => GToHtmlForm (l :*: r) where
+  gtoHtmlForm _ = do
+    gtoHtmlForm (Proxy @(l _))
+    gtoHtmlForm (Proxy @(r _))
+
+-- | ignore constructor & type metainfo
+instance GToHtmlForm a => GToHtmlForm (D1 r a) where
+  gtoHtmlForm _ = gtoHtmlForm (Proxy @(a _))
+instance GToHtmlForm a => GToHtmlForm (C1 r a) where
+  gtoHtmlForm _ = gtoHtmlForm (Proxy @(a _))
+
+-- | class for things which can be entered via html forms
+class ToHtmlForm a where
+  toHtmlForm :: Proxy a -> Html ()
+  default toHtmlForm :: (GToHtmlForm (Rep a)) => Proxy a -> Html ()
+  toHtmlForm = genericToHtmlForm defaultOptions
+
+data HtmlFormOptions = HtmlFormOptions
+  { htmlFormAction           :: Maybe Text
+  , htmlFormMethod           :: Text
+  , htmlFormSubmitButtonText :: Text
+  }
+
+defaultOptions ::HtmlFormOptions
+defaultOptions = HtmlFormOptions Nothing "post" "Ok"
+
+instance Default HtmlFormOptions where
+  defaultValue = defaultOptions
+
+genericToHtmlForm :: (GToHtmlForm (Rep a2)) => HtmlFormOptions -> Proxy a2 -> Html ()
+genericToHtmlForm HtmlFormOptions{..} (Proxy :: Proxy a) =
+  form_ ((method_ htmlFormMethod) : maybeToList (fmap action_ htmlFormAction)) $ do
+    gtoHtmlForm (Proxy @(Rep a _))
+    input_ [type_ "submit", value_ htmlFormSubmitButtonText]