[Template-haskell] reifyDecl: syntax and semantics

[Simon Peyton-Jones]

| > --------------------
| > d1 =3D [d| data T =3D ... |]
| >
| > splice d1
|
| I definitely prefer your solution which just avoids reification, for
| one simple reason: it works !
| Question is: is there any case where reification is indispensable or =
is
| this trick general ? (and then why bother with reification ?)
|=20
| However, I'm still not very happy with this solution, because (again)
| thinking of a library working on the reified declarations, then the
| part above has to be written by the "user".

I think the main reason for reifyDecl etc is so that you can write =
ordinary Haskell declarations, and subsequently (in another module, =
even) write (reifyDecl T) to get the decl for T.   It is rather =
dependent on being able to *name* the thing whose decl you want; hence =
the difficulty with instance decls (which are anonymous) and defn vs =
type-signature for functions (where one name names two things).

As to the "language-construct" question, the difficulty is this. If =
reifyDecl has type=20
	reifyDecl :: String -> Decl

then the string would have to be looked up in some environment.  Just =
which environment?  The environment at which the 'reifyDecl' call is =
written?  Or when the reifyDecl monadic computation is performed =
(perhaps much later).  I guess the latter is what we want.  That means =
the Q monad must support looking up strings in an environment, something =
I've been meaning to add for some time.  For example, suppose we say in =
current TH

	foo (reifyDecl T)

where T is some data type

	data T =3D MkT String

Then 'foo' might walk over the constructors of T, and over the =
constructor's argument type(s), and might ultimately come across a type =
constructor "String" in that argument type.   At that point, foo might =
want to reify the defn of String, so that it can see what manner of =
beast it might be.  And bingo there's that monadic look up again.=20

No one has actually asked for this yet, mind you, so it's kind of =
vaguely on my list.

Simon