I mean if `insert :: a -> Container a -> Container a`, and I call it with `[e| insert 5 True |]`, the quote will fail.

The goal here is to generate `Fn f` patterns in the property lambda whenever the `UnboundVarE` is a function. For example, today if I am given this:

[e| law "length/map" (length as == length (map f as)) |]

the generated code will be

property $ \as f -> length as === length (map f as)

when I would prefer to generate

property $ \as (Fn f) -> length as === length (map f as)

which will have significantly better UX. I'm willing to write a bad typechecker for `Exp`s, but really hoping I won't have to.

Thanks!

On Wed, Mar 18, 2020 at 1:04 AM Matthew Pickering <matthewtpickering@gmail.com> wrote:
Good morning Sandy, thanks for your email.

I don't think that GHC will typecheck the quote until you splice it
in. What exactly do you mean that it fails if `b` is replaced with
something different?

What are you hoping to do with this information?

This reminds me a bit of the `qTypecheck` action I have implemented on
another branch -
https://gitlab.haskell.org/ghc/ghc/issues/17565#note_242199

Cheers,

Matt

On Wed, Mar 18, 2020 at 1:56 AM Sandy Maguire <sandy@sandymaguire.me> wrote:
>
> Hi all,
>
> I'm writing some TH code that should generate property tests. For example, the expression:
>
> $(generate [e| law "idempotent" (insert a (insert a b) == insert a b) |])
>
> should generate the code
>
> property $ \a b -> insert a (insert a b) === insert a b
>
> I do this by looking for UnboundVarEs in the Exp returned by the [e| quote, and binding them in a lambda. All of this works.
>
> However, now I'm trying to get the inferred types of `a` and `b` in the above. GHC clearly is typechecking the quote, since it will fail if I replace `b` with something nonsensical. Is there some existent way to get the inferred type of an UnboundVarE --- ideally without reimplementing the typechecker?
>
> Thanks!
> Sandy
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