Stealing Impl from Rust

29 Jul 2021

With the new OverloadedRecordDot language extension, we can use the . character to access stuff on records.

{-# language OverloadedRecordDot #-}

data User = User { name :: String }

main :: IO ()
main = do
    let user = User { name = "Matt" }
    putStrLn user.name

This is syntax sugar for the following code:

import GHC.Records

data User = User { name :: String }

instance HasField "name" User String where
    getField (User n) = 
        n

main :: IO ()
main = do
    let user = User { name = "Matt" }
    putStrLn (getField @"name" user)

As it happens, we can add fields to a record.

{-# language OverloadedRecordDot #-}

data User = User { name :: String }

instance HasField "age" User Int where
    getField user = 
        32

main :: IO ()
main = do
    let user = User { name = "Matt" }
    print user.age

This works, though it’s a bit boring.

It’s much more useful to have, say, virtual fields.

data User 
    = User
    { firstName :: String
    , lastName :: String
    }

instance HasField "name" User String where
    getField user = 
        unwords [user.firstName, user.lastName]

This gives us a “virtual field,” which can allow us to refactor code that depends on the record field in neat ways!

Methods

So, those types, they don’t have to be ordinary values. They can be methods.

Or, y’know, functions, whatever, it’s all the same in Haskell.

instance HasField "greet" User (String -> IO ()) where
    getField self message = do
        putStrLn $ concat [message, ", ", self.name, "!"]

main :: IO ()
main = do
    let user = User { firstName = "Matt", lastName = "Parsons" }
    user.greet "UhhhHHH Excuse me WTF"

This prints out UhhhHHH Excuse me WTF, Matt Parsons!. Which is pretty cool.

`impl`

Rust has a keyword impl, which is used in two ways:

Adding methods to a type.
Adding a trait to a type.

The linked docs tell the whole story, just about.

I don’t know about you but I want nicer syntax than all the instance HasField stuff. I wrote a little library that should do this:

data User = User { name :: String }

impl ''User [d|

    greet :: String -> IO ()
    greet message = do
        putStrLn $ concat [message, ", ", self.name]
|]

It’s relatively straightforward. In pseudocode, it’s implemented like this:

impl :: Name -> Q [Dec] -> Q [Dec]
impl tyName qds = do
    decs <- qds
    let
        namesTypesExprs :: [(String, Type, Exp)]
        namesTypesExprs =
            getTypesAndExprs decs

    instances <- for namesTypesExprs $ \(name, typ, exp) -> do
        [d|
            instance HasField $(name) $(tyName) $(typ) where
                getField self = $(exp)
        |]
    pure (concat instances)

Unfortunately, I ran into a bit of a blocking issue, namely that GHC does not support OverloadedRecordDot in TemplateHaskell QuasiQuotes yet. While I can work around it, I’d rather not bother until OverloadedRecordDot is fully supported by GHC.

The Dealbreaker

There’s no polymorphism allowed.

Like, at all.

You can’t write:

instance (Show a) => HasField "myPrint" User (a -> IO ()) where
    getField self a = do
        putStrLn (show a)

This fails the functional dependencies. You can’t write methods generic in MonadIO m => HasField User (String -> m ()) either.

The functional dependencies seem pretty reasonable:

class HasField sym r a | sym r -> a where
    getField :: r -> a

This means that the types sym and r uniquely determine the a type - or, that if you know what sym and r are, then you always know exactly what a is. Since users of our isntance are able to select things like IO, ReaderT () IO, and StateT Int IO for this, you can’t uniquely determine the result type just based on the answer.

Seems like ImpredicativeTypes should work here, but they apparently don’t.

instance HasField "myPrint" User (forall a. Show a => a -> IO ()) where ...

This fails with a syntax error, due to the way OverloadedRecordDot affects GHC’s parser.

Bugs for the bug god!!

instance HasField "myPrint" User (forall a . Show a => a -> IO ()) where ...

This fails because it is an illegal polymorphic type.

Illegal Polymorphism

But wait - this is an impredicative type. ImpredicativeTypes was a deprecated language extension, but I recall hearing that we landed support for them with Quick Look Impredicativity. And, in GHC 9, we have a proper ImpredicativeTypes behavior! We definitely are paying a big cost for it - the Simplify Subsumption proposal gives no practical benefit to programmers except that it gives additional power to Quick Look Impredicativity.

Unfortunately, enabling ImpredicativeTypes doesn’t make this work - GHC still deems the above an illegal polymorphic type. It turns out, you can’t put an impredicative type in an instance at all.

Oh well.

BREAKING NEWS

Okay, so I posted this, and was immediately offered a Prime Tip by Sandy Maguire. Apparently Richard Eisenberg has published a video stating how to defeat this. The answer is to demand the constraint in the context.

So we can write myPrint like this:

instance 
    ( Show a 
    , HasField "myPrint" User (a -> IO ())
    )
  => 
    HasField "myPrint" User (a -> IO ()) 
  where
    getField self a = do
        putStrLn $ concat [self.name, " says: ", show a]

That let’s us write code like this:

go :: IO ()
go = do
    let user = User { name = "Matt" }
    user.myPrint 'a'
    user.myPrint 3

Which evaluates like this:

$> go
Matt says: 'a'
Matt says: 3

Nice!

Deal unbreaker.

Yet Another Update

Reddit user /u/WhisterPayer implemented this plugin to accomplish this! Awesome.