Sometimes I feel the need to selectively allow or disallow alternatives

This seems like the kind of lightweight property that Liquid Haskell (a GHC plugin) is especially suited for. Here's a small example of "at some places in the program we want the type system to enforce that only the constructor LeftFoo can be used." data Foo = LeftFoo !Int | RightFoo !Char {-@ measure isLeftFoo @-} isLeftFoo :: Foo -> Bool isLeftFoo LeftFoo{} = True isLeftFoo _ = False {-@ takesOnlyLeftFoo :: {x:Foo | isLeftFoo x} -> Int @-} takesOnlyLeftFoo :: Foo -> Int takesOnlyLeftFoo (LeftFoo i) = i I've taken Olaf's hypothetical Foo and swapped in Int for A and Char for B, defined a function *isLeftFoo* that returns True for LeftFoo-constructed Foos, and I call *isLeftFoo* in the type of a function that is meant to only be called with LeftFoo values. This syntax in the mushroom-at-symbol comments is a refinement type. It has the same structure as the haskell type `Foo -> Int` but adds a precondition on the input that *isLeftFoo* returns True. When you compile your project with Liquid Haskell, the precondition will be checked at compile time by checking that callers establish the fact on any value they pass into *takesOnlyLeftFoo*.

On 01-09-22 16:33, Dominik Schrempf wrote:

Chiming in here, maybe I missed something. Liquid Haskell is great, but in this case, in my opinion, easier is better:

newtype LeftFoo = LeftFoo !Int

newtype RightFoo = RightFoo !Char

data Foo = LFoo LeftFoo | RFoo RightFoo – Probably with strictness annotations.

isLeftFoo :: Foo -> Bool isLeftFoo (LFoo _) = True isLeftFoo _ = False

takesOnlyLeftFoo :: LeftFoo -> Int takesOnlyLeftFoo (LeftFoo i) = i

Dominik

Chiming in here too. I do this all the time if the subpart is to be used on 2 or more places. Easiest way to do this without extensions. -- Rubén. (pgp: 1E88 3AC4 89EB FA22)

In my real use case, things are not as simple as the example Foo, of course. There are fields of the same type, distributed across the choices, but some are optional in some choices and mandatory in others. Rank-1 parameters permit a range including, but not limited, to Indentity -- mandatory Maybe -- optional Const () -- missing Const Void -- constructor disallowed e.g. data Bar left right = LeftBar A (left O) | RightBar A B (right O) type OptionalLeftMandatoryRight = Bar Maybe Identity -- encodes: "If B is present so must be O, but if not, O is optional". type OnlyA = Bar (Const ()) (Const Void) -- isomorphic to A getO :: Applicative f => Bar f Identity -> f O getO (LeftBar _ o) = o getO (RightBar _ _ (Identity o)) = pure o getA :: Bar left right -> A getA (LeftBar a _) = a getA (RightBar a _ _) = a With distinct data types for LeftBar and RightBar, the functions getO, getA in their generality would only be possible via type classes. In some sense, the type parameters act as an explicit type class dictionary. I believe this is in the spirit of Trees that Grow, where the combination (left,right) = (Maybe,Identity) would be encoded in a type family. Olaf On Thu, 2022-09-01 at 22:33 +0200, Dominik Schrempf wrote:

Chiming in here, maybe I missed something. Liquid Haskell is great, but in this case, in my opinion, easier is better:

newtype LeftFoo = LeftFoo !Int

newtype RightFoo = RightFoo !Char

data Foo = LFoo LeftFoo | RFoo RightFoo – Probably with strictness annotations.

isLeftFoo :: Foo -> Bool isLeftFoo (LFoo _) = True isLeftFoo _ = False

takesOnlyLeftFoo :: LeftFoo -> Int takesOnlyLeftFoo (LeftFoo i) = i

Dominik

Patrick L Redmond via Haskell-Cafe writes:

...

Sometimes I feel the need to selectively allow or disallow alternatives

This seems like the kind of lightweight property that Liquid Haskell (a GHC plugin) is especially suited for. Here’s a small example of “at some places in the program we want the type system to enforce that only the constructor LeftFoo can be used.”

data Foo = LeftFoo !Int | RightFoo !Char

{-@ measure isLeftFoo @-} isLeftFoo :: Foo -> Bool isLeftFoo LeftFoo{} = True isLeftFoo _ = False

{-@ takesOnlyLeftFoo :: {x:Foo | isLeftFoo x} -> Int @-} takesOnlyLeftFoo :: Foo -> Int takesOnlyLeftFoo (LeftFoo i) = i

I’ve taken Olaf’s hypothetical Foo and swapped in Int for A and Char for B, defined a function isLeftFoo that returns True for LeftFoo-constructed Foos, and I call isLeftFoo in the type of a function that is meant to only be called with LeftFoo values.

This syntax in the mushroom-at-symbol comments is a refinement type. It has the same structure as the haskell type `Foo -> Int` but adds a precondition on the input that isLeftFoo returns True. When you compile your project with Liquid Haskell, the precondition will be checked at compile time by checking that callers establish the fact on any value they pass into takesOnlyLeftFoo.

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