RE: Problems with Template Haskell

I'd be interested to know what others think.

A big story for Template Haskell is that many type errors are found when
you *define* the macro (= template function) rather than when you *call*
the macro.  In TestSplice2 you had 
x = $(if False
          then
            [d|
            foo = bar
            |]
        else
          [d|
            d = 2
           |]
       )

This is rather artificial example; typically the condition will be
computed.  If TH didn't complain about the absence of 'bar', you might
well get an error when you splice x, maybe months later.

Still, in your cpp-like application, I guess your story is that the
condition might evaluate to True only if the system configuration was
such that bar was in scope.  If the condition evaluates to False, then
bar really might not be available.

OK, in TH (version 2 -- CVS HEAD) you want dynamic binding. You can say

        $(if ... then [|d foo = $(dyn "bar") |] .. )

Now you won't get any type complaint, and "bar" will get looked up at
the splice site of x.  (see
http://research.microsoft.com/~simonpj/tmp/notes2.ps)

Does that help?

Simon

| -----Original Message-----
| From: George Russell [mailto:ger@xxxxxx]
| Sent: 04 December 2003 11:42
| To: glasgow-haskell-bugs@xxxxxxxxxxx
| Cc: Simon Peyton-Jones
| Subject: Problems with Template Haskell
| 
| Template Haskell seems to be type-checking some quasi-quotes, even
when they
| are not going to be used.  This is of course a terrible nuisance,
since it
| means it can't be used to work around interface incompatibilities
between
| libraries for different versions of GHC (such as the recent change in
| RegexString.matchRegexAll's type).  Maybe I will have to go back to
using
| cpp ...
| 
| For example, the attached file fails to compile.
| 
| 
| # ghc TestSplice.hs -c -fglasgow-exts
|  >
|  > TestSplice.hs:7:
|  >     Couldn't match `f a' against `Bool'
|  >       Expected type: f a
|  >       Inferred type: Bool
|  >     In the second argument of `fmap', namely `True'
|  >     In the definition of `TestSplice.p': TestSplice.p = fmap id
True
| 
| This occurs for both ghc 6.0.1 and the recent snapshot  6.3.20031201
| 
| 
| 
| Another problem is that Template Haskell objects to undefined
variables in
| unused splices.  Thus if I replace "p = fmap id True" in the
| attached file by "foo = bar", I get "TestSplice2.hs:7: Variable not in
scope: `bar'"
|