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Adds custom type error for non-derivable instances of HasElmSorter #15

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Adds custom type error for non-derivable instances of HasElmSorter #15

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26 changes: 20 additions & 6 deletions src/Elm/Sorter.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,14 +8,17 @@
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}

module Elm.Sorter (Sorter, mkRecordSorter, mkCustom, HasElmSorter (..), render) where

import Data.Generics.Product.Fields (HasField')
import Data.Int (Int64)
import Data.Proxy
import Data.Text (Text, pack)
import Data.Text (pack)
import qualified Data.Text as T
import Elm.Common (letIn)
import GHC.Generics
import GHC.TypeLits
Expand All @@ -24,9 +27,9 @@ import Text.PrettyPrint.Leijen.Text hiding ((<$>))
data Sorter
= Alphabetical
| Increasing
| ByField Text Sorter
| ByNewtype Text Sorter
| Custom Text
| ByField T.Text Sorter
| ByNewtype T.Text Sorter
| Custom T.Text
deriving (Eq, Show)

render :: Sorter -> Doc
Expand Down Expand Up @@ -59,6 +62,17 @@ class HasElmSorter a where
class GenericElmSorter f where
genericElmSorter :: f a -> Sorter

instance
{-# OVERLAPPABLE #-}
( TypeError
( Text "ElmSorter only has default instances for newtypes."
:$$: Text "Perhaps you should use `mkRecordSorter` or `mkCustom` instead?"
)
) =>
GenericElmSorter a
where
genericElmSorter = undefined

Comment on lines +65 to +75
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this seems reasonable as long as this is the lowest-possible priority instance (which it should be, seeing as it's as general as possible)

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It is, yes

instance
(HasElmSorter interior, KnownSymbol consName) =>
GenericElmSorter
Expand Down Expand Up @@ -134,7 +148,7 @@ instance HasElmSorter Int64 where
instance HasElmSorter Double where
elmSorter _ = Increasing

instance HasElmSorter Text where
instance HasElmSorter T.Text where
elmSorter _ = Alphabetical

instance HasElmSorter String where
Expand All @@ -143,5 +157,5 @@ instance HasElmSorter String where
mkRecordSorter :: forall (field :: Symbol) record type_. (HasField' field record type_, KnownSymbol field, HasElmSorter type_) => Proxy record -> Sorter
mkRecordSorter _ = ByField (pack $ symbolVal (Proxy :: Proxy field)) (elmSorter (Proxy :: Proxy type_))

mkCustom :: Text -> Sorter
mkCustom :: T.Text -> Sorter
mkCustom = Custom