Overloaded.Plugin

Overloaded plugin, which makes magic possible.

Documentation

plugin :: Plugin

Overloaded plugin.

To enable plugin put the following at top of the module:

{-# OPTIONS -fplugin=Overloaded -fplugin-opt=Overloaded:Symbols #-}

At least one option is required, multiple can given either using multiple -fplugin-opt options, or by separating options with colon:

{-# OPTIONS -fplugin=Overloaded -fplugin-opt=Overloaded:Symbols:Numerals #-}

Options also take optional desugaring names, for example

{-# OPTIONS -fplugin=Overloaded -fplugin-opt=Overloaded:Labels=Data.Generics.ProductFields.field #-}

to desugar OverloadedLabels directly into field from generics-lens (no need to import orphan instance!)

Supported options

• Symbols desugars literal strings to fromSymbol @sym
• Strings works like built-in OverloadedStrings (but you can use different method than fromString)
• Numerals desugars literal numbers to fromNumeral @nat
• Naturals desugars literal numbers to fromNatural nat (i.e. like fromString)
• Chars desugars literal characters to fromChars c. Note: there isn't type-level alternative: we cannot promote Chars.
• Lists is not like built-in OverloadedLists, but desugars explicit lists to cons and nil
• If desugars if-expressions to ifte b t e
• Labels works like built-in OverloadedLabels (you should enable OverloadedLabels so parser recognises the syntax)
• TypeNats and TypeSymbols desugar type-level literals into FromNat and FromTypeSymbol respectively.

Known limitations

• Doesn't desugar inside patterns

RecordFields

WARNING the type-checker plugin is experimental, it's adviced to use

{-# OPTIONS_GHC -ddump-simpl #-}

to avoid surprising segfaults.

Usage

{-# OPTIONS -fplugin=Overloaded -fplugin-opt=Overloaded:RecordFields #-}

Implementation bits

See Note [HasField instances] in ClsInst, the behavior of this plugin is similar.

The HasField class is defined in GHC.Records.Compat module of record-hasfield package:

class HasField {k} x r a | x r -> a where
    hasField :: r -> (a -> r, a)

Suppose we have

data R y = MkR { foo :: [y] }

and foo in scope. We will solve constraints like

HasField "foo" (R Int) a

by emitting a new wanted constraint

[Int] ~# a

and building a HasField dictionary out of selector foo appropriately cast.

Idiom brackets from TemplateHaskellQuotes

{-# LANGUAGE TemplateHaskellQuotes #-}
{-# OPTIONS_GHC -fplugin=Overloaded -fplugin-opt=Overloaded:IdiomBrackets #-}

data Tree a
    = Leaf a
    | Branch (Tree a) (Tree a)
  deriving (Show)

instance Functor Tree where
    fmap f (Leaf x)     = Leaf (f x)
    fmap f (Branch l r) = Branch (fmap f l) (fmap f r)

instance Traversable Tree where
    traverse f (Leaf x)     = [| Leaf (f x) |]
    traverse f (Branch l r) = [| Branch (traverse f l) (traverse f r) |]