Proposal: Overloaded Quotes for Template Haskell
simonpj at microsoft.com
Thu Jan 5 19:08:36 CET 2012
I'm sorry Michael, I just don't get it. Maybe I'm just being slow.
One difficulty is that top-level splices (and hence, recursively) nested splices, must be run in the typechecker monad; and TH knows nothing of that monad. That's why the Quasi class exists in the first place; it says exactly what the "host monad" (the typehcecker in this case) must support. By moving to ST you are fixing the monad, and I just don't see how to make that work.
Perhaps the thing to do is to write some typing judgements, as in the TH paper, or to implement a prototype, or to write up a design in more detail. Template Haskell always does my head in.
Sorry not to be more helpful
| -----Original Message-----
| From: Michael D. Adams [mailto:mdmkolbe at gmail.com]
| Sent: 31 December 2011 19:59
| To: Simon Peyton-Jones
| Cc: libraries at haskell.org
| Subject: Re: Proposal: Overloaded Quotes for Template Haskell
| On Sat, Dec 31, 2011 at 7:55 AM, Simon Peyton-Jones
| <simonpj at microsoft.com> wrote:
| > | > And indeed there is such a function: it's called runQ.
| > | >
| > | > So I think what you want is already available.
| > |
| > | If there are no splices in the quote then, yes, that is sufficient.
| > | However, as there is no function of type "forall m. Quasi m => m Exp
| > | -> Q Exp", the contents of all splices must be of type "Q Exp". Thus
| > | in the expression "[| ... $( foo ) ... |]" there is no way for foo to
| > | modify the state of the memoization table.
| > I'm sorry, I don't understand what you say here.
| > You started your post by saying that you want quote to be of type
| > forall m. Quasi m => m Exp
| > But they already are! (With a newtype wrapper.)
| The constructor of that newtype wrapper is *not* exported by the
| Template Haskell libraries. The destructor *is* exported in the form
| of "runQ" (which is equivalent to "unQ" (and I don't know why the two
| names)), but the constructor "Q" is not.
| However, while it may look like exporting "Q" is sufficient to get
| what I want, that doesn't quite work.
| To see why it doesn't work, suppose we have:
| - "c :: StateT S Q Exp -> StateT S Q Exp" and
| - "e :: StateT S Q Exp" and
| - "instance Quasi (StateT s Q)".
| Now, consider "c (runQ [| ... $(Q (e)) ... |])". There will be a type
| error in the application of "Q" to "e" since "forall m. Quasi m => m
| Exp" is not an instantiation of "State S Q Exp".
| However, if the types of quotes and splices are generalized as I
| proposed, then "c [| ... $(e) ... |]" will type check just fine and
| achieve the effect that I'm after.
| Another way to think of this distinction is to the quotes as
| functions. The function "\e = [| ... $e ... |]" has type "Q Exp -> Q
| Exp". On the other hand the function "\e -> runQ [| ... $(Q e) ...
| |]" has type "(forall m. Quasi m => m Exp) -> (forall m. Quasi m => m
| Exp)". Note that the two "m" do not unify. Under my proposal "\e ->
| [| ... $e ... |]" would have type "forall m. Quasi m => m Exp -> m
More information about the Libraries