[Template-haskell] Records and generating splices

[Simon Peyton-Jones]

Template Haskell is indeed poorly documented.

I keep hoping that someone will undertake the task of writing better
documentation, since that is something that doesn't have to be done at
GHC HQ.

Meanwhile, your best bets are 
a) the user manual
http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell
.html
including the "notes2.ps" pointed to from the user manual, and the
original paper
b) the Template Haskell mailing list... but no one has replied to your
message.  You could try ghc-users for a wider audience
c) when it comes to the exact name for each constructor and function,
you need the online library documentation, but I think you've found that
already.

It's not clear from your message, but I think you want to do something
like what is described in Section 8 of
http://research.microsoft.com/~simonpj/tmp/notes2.ps.  If so, I'm afraid
that isn't implemented yet, and (somewhat to my surprise) no one has
been yelling for it.  It's not trivial to implement unfortunately.
It's also not essential: you can drop down to the explicit-constructor
level described in the original paper.


Simon

| -----Original Message-----
| From: template-haskell-bounces at haskell.org
[mailto:template-haskell-bounces at haskell.org] On
| Behalf Of Rene de Visser
| Sent: 08 July 2005 13:40
| To: template-haskell at haskell.org
| Subject: [Template-haskell] Records and generating splices
| 
| Hello,
| 
| In the following code I wish to generate the invidual accessors, i.e.
the
| add_rel1, add_rel2, etc.
| 
| I have tried using
| 
| test = [| add_rel1 value = modify (\db -> db{ rel1 = Set.insert value
(rel1
| db)}) |]
| 
| with some splicing, but it does not seem possible to splice in the
rel1
| before the '='.
| Also how do I splice in the add_rel1?
| 
| According to the documentation it is only possible to splice in
complete
| expressions. This seems quite limiting. Does this mean you always have
to
| construct the expressions by hand?
| 
| The library documention seems very thin. Its hard to tell how to
create a
| record accessor and setter from the haddock documentation.
| 
| Note that I have no experience with template haskell.
| 
| It would also be nice to generate the type declaration, but I am
finding
| this even more difficult.
| 
| Can any one provide some example code to do this.
| 
| Rene.
| 
| 
| {-# OPTIONS -fglasgow-exts #-}
| module DataBase where
| 
| import Control.Monad.State
| import qualified Data.Set as Set
| -- Does this really need to be extensible???
| -- Entries need to be based on sets, or something similar...
| data SmallDB = SmallDB { rel1 :: Set.Set String
|                        , rel2 :: Set.Set Integer } deriving Show
| 
| emptyDB = SmallDB Set.empty Set.empty
| 
| add_rel1 :: (MonadState SmallDB m) => String -> m ()
| add_rel1 value = modify (\db -> db{ rel1 = Set.insert value (rel1
db)})
| 
| add_rel2 :: (MonadState SmallDB m) => Integer -> m ()
| add_rel2 value = modify (\db -> db{ rel2 = Set.insert value (rel2
db)})
| 
| 
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