| 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:[email protected]] On Behalf Of Izzy
| Meckler
| Sent: 22 February 2015 06:12
| To: [email protected]
| 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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| [email protected]
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