Flag, a tagged Bool

Posted on 2019-03-21 by Oleg Grenrus engineering

This posts complements two other recent blogs: Code smell: Boolean blindness by Thomas Tuegel and Ceci n'est pas un default by Guillaume Allais.

Thomas writes

The popular term “boolean blindness” refers to the information lost by functions that operate on Bool when richer structures are available

and indeed, in one project I wrote a function with a very horrible type:

    :: UseColors
    -> Bool
    -> Bool
    -> PlanJson
    -> Bool
    -> Bool
    -> IO ()

I could add comments, but those are unchecked by a compiler. I could use Maybe or some other richer type, but these Bools are coming from command line arguments, which are just single bit of information. What else can I do?

Phantom types is one possible solution.

#Phantom type

After small module header...

{-# LANGUAGE KindSignatures, DataKinds #-}
{-# LANGUAGE FunctionalDependencies, FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables, DeriveAnyClass #-}
-- | Because 'Flag' constructor isn't exposed,
-- we have to explicitly 'toFlag' and 'fromFlag'.
-- That way it's less likely we mix up bare Booleans.
module Flag (
    -- * Flag
    Flag, toFlag, fromFlag,
    -- * HasDefault
    -- * optparse-applicative
    ) where

import           Control.Applicative ((<|>))
import           Data.Proxy (Proxy (..))
import           Data.Semigroup (Semigroup (..))
import           Data.Singletons.Bool
import qualified Options.Applicative as O

... we can define Flag:

newtype Flag t = MkFlag Bool

toFlag :: t -> Bool -> Flag t
toFlag _ = MkFlag

fromFlag :: t -> Flag t -> Bool
fromFlag _ (MkFlag b) = b

A Flag is just a Bool. But Flag Foo and Flag Bar are different types!

As the module export list already revealed, the MkFlag constructor is not exported. The only way to construct and match on Flag is to use toFlag and fromFlag functions. Both functions take an additional t argument. The idea is to define a data type per flag and use the single value as a proxy for a type.

data ShowBuiltin = ShowBuiltin
data ShowGlobal  = ShowGlobal
data TopoReverse = TopoReverse
data ShowFlags   = ShowFlags

We define a nullary constructor, so the type can act as its own Proxy. Using Flag, we can make doTopo signature much more comprehensible:

    :: UseColors
    -> Flag ShowBuiltin
    -> Flag ShowGlobal
    -> PlanJson
    -> Flag TopoReverse
    -> Flag ShowFlags
    -> IO ()
doTopo ... =
    -- here the redundancy is good, it will help catch errors
    if fromFlag ShowBuiltin showBuiltin
    then ...
    else ...

Now the type signature is much more self-explanatory, and it's impossible (or at least quite hard) to mixup different flags. And note, this is completely Haskell98 solution. So far we didn't need any extensions.

#An alternative: record type

Another alternative would been to define a helper structure,

data TopoOpts = TopoOpts
    { toUseColors   :: UseColors
    , toShowBuiltin :: Bool
    , toShowGlobal  :: Bool
    , toPlanJson    :: PlanJson
    , toTopoReverse :: Bool
    , toShowFlags   :: Bool

especially with RecordWildCards language extension, it's convinient to use. Also with this approach it's harder to mixup different Bools. Both Flag and a record name Booleans.

For whoever is wondering, the UseColors type is a three way setting: always, never or auto, so I wrote a special type for it. One could write own type for each Boolean flag too. I didn't explore that alternative. We'd need to case, as there aren't natural candidate to proxy in fromFlag variant (which can be made with Generics: but one have to be careful to define values in order as in data Bool = False | True!).

#Make phantom relevant

It depends which approach, Flag or a record, is more convinient (or even both together!). Flag approach let us do some nice extra stuff, thanks to a type-level tag. But we have to departure the Haskell98-land. Inspired by Guillaume's post, we can do a little of dependent-ish programming. I use my singleton-bool library here.

First we define HasDefault class.

-- | Has default boolean value.
class SBoolI def => HasDefault (def :: Bool) t | t -> def

-- | Flag with a default value.
def :: forall t def. HasDefault def t => t -> Flag t
def t = toFlag t (reflectBool (Proxy :: Proxy def))

The FunctionalDependencies (and not TypeFamilies) are used so we can use DeriveAnyClass extension:

data MyFlag = MyFlag deriving (HasDefault 'True)

Now the phantom type isn't really-really phantom anymore, HasDefault constraint makes it "relevant".

#An example: using with optparse-applicative

We can use HasDefault to implicitly wire-in things, for example when making optparse-applicative parsers. Remember, Flags often come from command line arguments.

I'd like to use specialised parser for Show* flags. Now we can use the fact that we know the default value: And the default value is specified close to flag declaration (visible in Haddocks!), and not hidden somewhere in the command line parser code. That's a win. So we can write quite generic showHide parser combinator:

showHide :: HasDefault def t => t -> String -> String -> O.Parser (Flag t)
showHide t n d =
    O.flag' (toFlag t True) (O.long ("show-" ++ n) <> O.help d)
    <|> O.flag' (toFlag t False) (O.long ("hide-" ++ n))
    <|> pure (def t)

Note: we can go even further: we could define a HasParser class and with a little DerivingVia magic parsers could be completely derivable. I leave that as an exercise :)

Another use case: super power switch. We can make GHC verify that the default value is False, so if we decide to change the default, we won't forget to fix the cli parsers:

switchA :: HasDefault 'False t => t -> String -> String -> O.Parser (Flag t)
switchA t n d = fmap (toFlag t) $ O.switch $ O.long n <> O.help d

Or we can make switch flip automatically:

switchB :: HasDefault def t => t -> String -> String -> O.Parser (Flag t)
switchB t n d = fmap (toFlag t) $ O.flag v (not v) $ O.long n' <> O.help d
    n' = if v then "no-" ++ n else n
    v  = fromFlag t (def t)

Careful reader would notice that showHide, switchA and switchB are string-ly typed. But that again, depends; not bad in this case. In the real codebase showHide is used like:

<$> showHide ShowBuiltin "builtin" "Show / hide packages in global (non-nix-style) package db"
<*> showHide ShowGlobal  "global"  "Show / hide packages in nix-store"

Wrapping in additional newtype won't help; adding IsString instance would make newtype not any better than plain String. On the other hand, if we use DerivingVia to derive parsers, whole problem would go away.


To conclude, phantom types are quite useful, and if you spice them with a little of type-level programming, you won't need to write boilterplate and errorprone code.

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