Hello Haskell world.
One well-known missfeature of TemplateHaskell is it's «untypedness». I
mean, that if you have say an expression (Exp or Q Exp), you do not know
it's type (does it represent String or Int or whatever). And GHC does
not know too. So, one could easily construct bad-typed expressions using
TH, and GHC will complain only on generated code, not on TH code.
Such a missfeature was a reasoned decision, because adding some typing
into TH will add much complexity to compiler and to language.
As far as can I see, using features of last GHC one could write typed
TH library relatively easily, and saving backwards compatibility.
For example, now we have Q monad and Exp type in "template-haskell"
package. Let's imagine some new package, say "typed-template-haskell",
with new TQ monad and new polymorphic type Exp :: * -> *. Using last
GHC's features, one will easily write something like "expr :: Exp
String", which will mean that "expr" represents a string expression. And
we will need a new function, say runTQ :: TQ a -> Q a (or some more
complicated type), which will turn TypedTemplateHaskell's constructs
into plain TH.
One question will be on quotations and antiquotations: they are
implemented to work untyped TH constructs, how to implement
compatibility with new typed TH? We could add some support of them to
GHC... Or, maybe define a phantom type (say, Any), and some conversion
functions, say untyped :: Language.Haskell.TH.Syntax.Exp ->
Language.Haskell.TH.Typed.Syntax.Exp Any.
Main idea is: seems new GHC enables us to implement "typed TH" without
modifying GHC itself, only by writing a library. Is it so?
Any further thoughts?
With best regards,
Ilya Portnov.
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