Writing a subtype function

[Simon Peyton Jones]

| I’m at a loss as to how to use TcUnify.tcSubType. My goal is to write a
| function of type
| 
| Type -> Type -> Ghc Bool

I assume you are talking here about the GHC API?  tcSubType is usually called during type inference, and it rightly does not return a Bool. Why? Because we may not know whether it will succeed or fail until we have walked the entire syntax tree of the program; perhaps there is a bit of code that forces a crucial unification.

So tcSubType returns a `HsWrapper`, which you can use to wrap a term of type t1, to produce a term of type t2.  

It ALSO emits some constraints (in the monad) which can be solved later.  If the constraints are soluble, we have a proof that t1 is a subtype of t2. If not, we don’t.

In the context of the GHC API you probably need something like

do { (_wrapper, constraints) <- captureConstraints (tcSubType t1 t2)
   ; tcSimplifyTop constraints }

The 'captureConstraints' grabs the constraints generated by tcSubType; the tcSimplifyTop tries to solve them and reports errors.   If you don’t want to report errors, you can doubtless use some variant of tcSimplifyTop.

I hope this helps

Simon

| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Izzy
| Meckler
| Sent: 22 February 2015 06:12
| To: ghc-devs at haskell.org
| Subject: Writing a subtype function
| 
| Hi all,
| 
| I apologize if this isn’t the right place for this sort of question, but
| I’m at a loss as to how to use TcUnify.tcSubType. My goal is to write a
| function of type
| 
| Type -> Type -> Ghc Bool
| 
| which checks whether the first argument is a subtype of the second. I
| assume this is possible to do using TcUnify.tcSubType and
| runTcInteractive, but it’s not clear to me how. Any pointers here would
| be greatly appreciated.
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