[Haskell-cafe] selectively allow alternatives in a sum type

[Patrick L Redmond]

I made a TH function earlier this year to produce types for constructors:

data Foo = LeftFoo !Int | RightFoo !Char
    deriving Show

$(constructorType 'LeftFoo)
deriving instance Show LeftFooC

In a repl you can see it generates a type with a single constructor, an
introducer, and an eliminator.

*Main> :browse
type Foo :: *
data Foo = LeftFoo !Int | RightFoo !Char
type LeftFooC :: *
data LeftFooC = LeftFooC Int
introLeftFooC :: Foo -> Maybe LeftFooC
elimLeftFooC :: LeftFooC -> Foo

*Main> introLeftFooC (RightFoo 'a')
Nothing

*Main> introLeftFooC (LeftFoo 123)
Just (LeftFooC 123)

*Main> elimLeftFooC $ LeftFooC 456
LeftFoo 456

If you find that useful, I ripped it out of my other repo and pasted it
here: https://gist.github.com/plredmond/5fd15fb388647b248de87693542d2adb

On Thu, Sep 1, 2022 at 3:54 PM Ruben Astudillo <ruben.astud at gmail.com>
wrote:

> 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)
>
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