Getting the inferred types of TH's UnboundVarEs

Richard Eisenberg rae at
Thu Mar 19 09:51:13 UTC 2020

Good to see you around, Sandy!

> On Mar 18, 2020, at 6:54 PM, Sandy Maguire <sandy at> wrote:
> I mean if `insert :: a -> Container a -> Container a`, and I call it with `[e| insert 5 True |]`, the quote will fail.

I don't observe this. Specifically, when I compile

> {-# LANGUAGE TemplateHaskellQuotes #-}
> module Bug where
> import Prelude ( Bool(..), undefined )
> data Container a
> insert :: a -> Container a -> Container a
> insert = undefined
> quote = [e| insert 5 True |]

GHC happily succeeds.

I think what you want, though, is reasonable: you want the ability to send an expression through GHC's type-checker. I think we'd need to extend TH to be able to support this, and it will be hard to come up with a good design, I think. (Specifically, I'm worried about interactions with top-level defined entities, whose types might not really be known by the time of splice processing.) This might all be worthwhile -- singletons would be able to be improved with this, for example -- but it's not cheap, sadly.

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