[Haskell-cafe] Counterintuitive ScopedTypeVariables behaviour

[Erik Hesselink]

On Mon, 17 Aug 2020 at 07:10, Anthony Clayden <anthony_clayden at clear.net.nz>
wrote:

> Hi Tom, I see nothing bizarre nor counterintuitive. Perhaps you need to retrain your intuitions ;-)
>
> > I have just noticed that with ScopedTypeVariables one can write the
> > following bizarre definition
> >
> >    -- Inferred signature:
> >    -- add :: Num a => a -> a -> a
> >    add (x :: a) (y :: b) = x + y
>
> What do you think is bizarre? GHC makes a reasonable attempt to use the tyvar names you put in a signature -- remembering that alpha-renaming means the name is not really significant. I can get `add :: Num c => c -> c -> c`, etc.
>
>
I can't speak for Tom, but to me the above feels very similar to writing
something like:

    add :: _ => a -> b -> _
    add x y = x + y

But that doesn't type check, since the types `a` and `b` are not the same,
but `+` needs them to be.

Erik
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