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Johannes Waldmann | 4 Oct 2005 14:26
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automatic instances for pretty printing and parsing

Dear all,

I use Text.PrettyPrint.HughesPJ and
Text.ParserCombinators.Parsec heavily so I made
some DrIFT rules for deriving the "obvious" instances for

class ToDoc  a where toDoc :: a -> Doc
class Reader a where readerPrec :: Int -> Parser a

http://141.57.11.163/cgi-bin/cvsweb/drift/src/UserRuleToDoc.hs?rev=1.5

Still I'd like to avoid external preprocessors (for hygienic reasons,
and because they do not seem to fit well with the Cabal model)
so I'd like to investigate whether I could get the same effect
with Template Haskell (in ghc) - and what this would cost.
Perhaps someone has solved a similar problem before.

On the other hand, HaXml does need DrIFT it seems.
--

-- 
-- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 --
---- http://www.imn.htwk-leipzig.de/~waldmann/ -------
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Tomasz Zielonka | 7 Oct 2005 15:12
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mkName problem

Hello!

The attached code shows a behaviour which I don't understand. This could
be a bug in TH or a misunderstanding on my side. The problem is that dynamically
bound Names made with mkName sometimes "don't see" the variable. See
the code for details. It could be a bit shorter, but I wanted to show why I cat't
use the 'var syntax. Try to --make T, then uncomment the marked line in T.hs.

Best regards
Tomasz


Attachment (T.hs): text/x-haskell, 234 bytes
Attachment (Mod.hs): text/x-haskell, 329 bytes
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Simon Peyton-Jones | 25 Oct 2005 11:47
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RE: Documentation Template Haskell

Christoph, and other TH users,

| > I have been waiting for a long while for more documentation
| > about the use of Template Haskell as it is currently implemented,
| > now ghc-6.4.1. The documentation on the web refers to several old

You're absolutely right.  Template Haskell is appallingly badly
documented.  The reason I have not done anything about it is that
there's very little traffic on the Template Haskell list (or any other,
about TH), and that has de-motivated me so far as improving TH is
concerned.  I tend to pay more attention to things that I think people
are actively using and care about.  

TH seems to have had quite a bit less impact than I originally thought
it might, given that it provides something qualitatively new.  But maybe
I'm wrong.  For example, I didn't know that it was important to you.
Maybe there are others.

In any case, as you may guess I find it difficult to find enough time to
do all the GHC related things that GHC's users would like.  That's good,
because it means GHC is being useful, but bad because some things don't
get done. 

 My current hope is to broaden the base of people who are willing to
contribute to GHC themselves, with support from me and Simon, so that
Simon and I aren't always on the critical path.

So here's an offer.  If you'd be willing to draft the documentation you
would like to see, I would be willing to 
	- review it, 
	- answer questions, and 
	- write parts where you don't know what to say. 
But you already know a lot, so you are well placed to write an outline.
Perhaps TH folk would be willing to help too?  RSVP to the TH list
(which I'm ccing).

I know this won't be quite the reply you were hoping for, but I hope
you'll consider it.

Simon

| -----Original Message-----
| From: Christoph Herrmann [mailto:herrmann <at> uni-passau.de]
| Sent: 22 October 2005 18:42
| To: Simon Peyton-Jones
| Subject: Re: Documentation Template Haskell
| 
| Dear Simon,
| 
| > Anything more than a list of library function types would do it.
| 
| It would be good if it contains some explanations. Recently
| I have found the following example on the mailing list but I can
| only guess what, e.g., "tupP" means.
| 
| sel' :: Int -> Int -> ExpQ
| sel' i n = lamE [pat] rhs
|     where pat = tupP (map varP as)
|           rhs = varE (as !! (i - 1))
|           as = map mkName [ ("a__" ++ show j) | j <- [1..n] ]
| 
| Looking up Google for "tupP TH" leads me to some pages about TH
| where I can either find some old example TH programs without "tupP"
| or a list of library functions where I can only get the type of
"tupP".
| What I need would be a few English words telling what tupP actually
does.
| 
| Also, I'm wondering about the TH function "tuple" below whether it is
| an old version, a plan for the future or even supposed to work with
| ghc-6.4.1 but I forgot some import declarations.
| 
| tuple :: Int -> Q Exp
| tuple n = [|\list -> $(tupE (exprs [|list|])) |]
|   where
|     exprs list = [infixE (Just (list::[Char]))
|                          (varE "!!")
|                          (Just (litE $ integerL (toInteger num)))
|                     | num <- [0..(n - 1)]]
| 
| 
| Christoph Herrmann wrote:
| > Dear Simon,
| >
| > I have been waiting for a long while for more documentation
| > about the use of Template Haskell as it is currently implemented,
| > now ghc-6.4.1. The documentation on the web refers to several old
| > versions of TH and most of the examples don't work any more,
| > e.g., the generation of a function which selects a component
| > from a tuple. I don't know which of the new features you proposed
| > are currently implemented.
| >
| > I need Template Haskell both for my work on metaprogramming
| > and for teaching. Could you please point me to information
| > about the current state? It doesn't matter for me how well it is
| > written, as far as the definitions are up-to-date. Also, some
examples
| > which deal with code construction and reification and which work
with
| > ghc-6.4.1 would be useful for me. Again, I do not care about how
they
| > look like. Anything more than a list of library function types would
do it.
| >
| > Many thanks in advance
| > --
| >  Christoph Herrmann
| >  University of Passau, Germany
| >
| >
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Simon Peyton-Jones | 25 Oct 2005 11:54
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RE: Documentation Template Haskell


| It would be good if it contains some explanations. Recently
| I have found the following example on the mailing list but I can
| only guess what, e.g., "tupP" means.
| 
| sel' :: Int -> Int -> ExpQ
| sel' i n = lamE [pat] rhs
|     where pat = tupP (map varP as)
|           rhs = varE (as !! (i - 1))
|           as = map mkName [ ("a__" ++ show j) | j <- [1..n] ]

tupP has type [Q Pat] -> Q Pat
http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell/L
anguage-Haskell-TH-Lib.html#v%3AtupP

It's a "smart constructor" for patterns, just like the other "smart
constructors" described in the original TH paper (section 6.3)
http://research.microsoft.com/%7Esimonpj/Papers/meta-haskell/index.htm
[In the paper, the type Expr is now ExpQ, or equivalently Q Exp.]

Does that help?

Simon
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Bulat Ziganshin | 25 Oct 2005 16:25

Re: RE: Documentation Template Haskell

Hello Simon,

Tuesday, October 25, 2005, 1:47:38 PM, you wrote:

SPJ> So here's an offer.  If you'd be willing to draft the documentation you

"TH can be used to automatically generate any Haskell code at the compile
time. This code will be compiled just the same as if it was
hand-written. This allows to create metaprograms - programs, which
itself generate Haskell programs, allowing to use programming
techniques not available in Haskell itself, such as macro
preprocessing, self-optimized algorithms, generic programming,
functions with variable parameters count and so on

TH code is written as ordinal Haskell functions, which generate AST
(abstract syntax tree) representing Haskell code. This AST represented
by type Exp. Moreover, these functions must function in special monad
Q, which provides abilities to generate unique identifiers for
code builded and to make queries about environment in which this function
is running.

Type Exp allows to represent any Haskell code:

data Exp = LitE Lit  -- representing literals
         | VarE Name -- representing variables
         .....

Type Lit, used in Exp, represents Haskell literals:
.....
-- description for other types

To make TH programming easier, the above-mentioned constructors are
duplicated by functions which accepts arguments lifted to Q monad and
return results in the same monad. For example, AppE constructor dubbed
by appE function. These functions are defined using types which is
also lifted to Q monad, such as ExpQ = Q Exp and so on.

To create unique identifiers for generated code you must use the
following functions: ....

You can also query about environment which runs TH function with the
following functions: ....

Also, to simplify building of ASTs, Template Haskell can automatically
translate ordinary Haskell code to this representation by using
[|...|]. This code has type ExpQ, so it is a computation which will
return Exp, representing code inside brackets, on execution in monad Q.
Code inside brackets can be any Haskell code, local identifiers will
be translated to code generating unique identifiers.

If you need to create expression of type Pat, use form [p| ... |],
type Dec - use [d| ... |].

Code, generated by TH functions, can be spilled to hand-written code
by using splice notation: $function or $(function parameters...). You
can use splices in any place where Haskell expression or declaration
can be written. Of course, the spliced expression must return value of
appropriate type - ExpQ or DecQ, respectively. You also can use spills
in definitions of TH functions, inside abovementioned brackets, making
some form of macrogeneration. In other cases, you must import TH
functions in order to spill it, because TH-generating code must be
already compiled in time where current module code isn't even
syntax-processed"

>END-OF-DOC

this doc doesn't include 'name and ''type, whose explanations i can't
recall. i also wrote TH tutorial, but don't finish it. You can see the
last edition as http://freearc.narod.ru/th.htm

one more suggestion - i think that we must include in GHC
distribution useful TH application libraries: printf, class derivation
(a la DrIFT)

--

-- 
Best regards,
 Bulat                            mailto:bulatz <at> HotPOP.com
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Johannes Waldmann | 25 Oct 2005 16:51
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Re: RE: Documentation Template Haskell

Bulat Ziganshin wrote:

> one more suggestion - i think that we must include in GHC
> distribution useful TH application libraries: printf, class derivation
> (a la DrIFT)

I'd love that. Are there any such libraries at the moment? Somewhere?
If there is some kind of blueprint, I could perhaps use it
http://www.haskell.org/pipermail/template-haskell/2005-October/000474.html
--

-- 
-- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 --
---- http://www.imn.htwk-leipzig.de/~waldmann/ -------
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Bulat Ziganshin | 25 Oct 2005 17:06

Re[2]: RE: Documentation Template Haskell

Hello Johannes,

Tuesday, October 25, 2005, 6:51:21 PM, you wrote:

JW> Bulat Ziganshin wrote:

>> one more suggestion - i think that we must include in GHC
>> distribution useful TH application libraries: printf, class derivation
>> (a la DrIFT)

JW> I'd love that. Are there any such libraries at the moment? Somewhere?

http://www.haskell.org/th/papers/Template_Haskell-A_Report_From_The_Field.ps

which is mentioned at http://www.haskell.org/th/

--

-- 
Best regards,
 Bulat                            mailto:bulatz <at> HotPOP.com
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Tomasz Zielonka | 26 Oct 2005 08:58
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Re: RE: Documentation Template Haskell

On 10/25/05, Simon Peyton-Jones <simonpj <at> microsoft.com> wrote:
> TH seems to have had quite a bit less impact than I originally thought
> it might, given that it provides something qualitatively new.  But maybe
> I'm wrong.  For example, I didn't know that it was important to you.
> Maybe there are others.

I also use TH, mostly to generate instances of some classes and
for things like interpolation (this is the word they use in Perl-world
for this - I've used Perl several years ago) of variables in String literals.
Probably not the most sophisticated uses, but they sure make writing
(some kinds of) code much easier.

There are probably many things that could  be done to make TH
more useful, and hopefully also more popular. Here are some I
thought about:
- allow to use TH to create rules for deriving instances of
  user defined type classes - so the end user doesn't have
  to know about TH - this is quite controversial, because on
  one hand it would make TH less visible, but on the other,
  library writers would be more willing to use it
- allow to use TH as a kind of preprocessor, so TH-preprocessed
  source can be loaded by other compilers
- bridge that gap between ADTs used in TH and haskell-src(-exts),
  so, for example, I could parse parts of String literals as Haskell.
- alternatively, allow to splice inside String literals ;-)
- allow to use syntaxes other than Haskell in quotations, for example,
  I would love to use Haskell to generate C++ (to improve Haskell/C++
  ratio in my code). In other words, make TH an aid in using DSLs.

>  My current hope is to broaden the base of people who are willing to
> contribute to GHC themselves, with support from me and Simon, so that
> Simon and I aren't always on the critical path.

I would like to clarify that I don't expect Simon and you to do any of
these. I am already amazed that you did so much :-)

Best regards
Tomasz
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Simon Peyton-Jones | 26 Oct 2005 11:26
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RE: RE: Documentation Template Haskell

Good start.  I'm not sure whether it would have enough detail to meet
Christoph's need, but let's ask him.  Maybe he can fill in some of the
blanks.

Simon

| -----Original Message-----
| From: template-haskell-bounces <at> haskell.org
[mailto:template-haskell-bounces <at> haskell.org] On
| Behalf Of Bulat Ziganshin
| Sent: 25 October 2005 15:25
| To: Simon Peyton-Jones
| Cc: Christoph Herrmann; template-haskell <at> haskell.org
| Subject: Re: [Template-haskell] RE: Documentation Template Haskell
| 
| Hello Simon,
| 
| Tuesday, October 25, 2005, 1:47:38 PM, you wrote:
| 
| SPJ> So here's an offer.  If you'd be willing to draft the
documentation you
| 
| "TH can be used to automatically generate any Haskell code at the
compile
| time. This code will be compiled just the same as if it was
| hand-written. This allows to create metaprograms - programs, which
| itself generate Haskell programs, allowing to use programming
| techniques not available in Haskell itself, such as macro
| preprocessing, self-optimized algorithms, generic programming,
| functions with variable parameters count and so on
| 
| TH code is written as ordinal Haskell functions, which generate AST
| (abstract syntax tree) representing Haskell code. This AST represented
| by type Exp. Moreover, these functions must function in special monad
| Q, which provides abilities to generate unique identifiers for
| code builded and to make queries about environment in which this
function
| is running.
| 
| Type Exp allows to represent any Haskell code:
| 
| data Exp = LitE Lit  -- representing literals
|          | VarE Name -- representing variables
|          .....
| 
| Type Lit, used in Exp, represents Haskell literals:
| .....
| -- description for other types
| 
| 
| To make TH programming easier, the above-mentioned constructors are
| duplicated by functions which accepts arguments lifted to Q monad and
| return results in the same monad. For example, AppE constructor dubbed
| by appE function. These functions are defined using types which is
| also lifted to Q monad, such as ExpQ = Q Exp and so on.
| 
| To create unique identifiers for generated code you must use the
| following functions: ....
| 
| You can also query about environment which runs TH function with the
| following functions: ....
| 
| 
| Also, to simplify building of ASTs, Template Haskell can automatically
| translate ordinary Haskell code to this representation by using
| [|...|]. This code has type ExpQ, so it is a computation which will
| return Exp, representing code inside brackets, on execution in monad
Q.
| Code inside brackets can be any Haskell code, local identifiers will
| be translated to code generating unique identifiers.
| 
| If you need to create expression of type Pat, use form [p| ... |],
| type Dec - use [d| ... |].
| 
| Code, generated by TH functions, can be spilled to hand-written code
| by using splice notation: $function or $(function parameters...). You
| can use splices in any place where Haskell expression or declaration
| can be written. Of course, the spliced expression must return value of
| appropriate type - ExpQ or DecQ, respectively. You also can use spills
| in definitions of TH functions, inside abovementioned brackets, making
| some form of macrogeneration. In other cases, you must import TH
| functions in order to spill it, because TH-generating code must be
| already compiled in time where current module code isn't even
| syntax-processed"
| 
| >END-OF-DOC
| 
| 
| 
| this doc doesn't include 'name and ''type, whose explanations i can't
| recall. i also wrote TH tutorial, but don't finish it. You can see the
| last edition as http://freearc.narod.ru/th.htm
| 
| one more suggestion - i think that we must include in GHC
| distribution useful TH application libraries: printf, class derivation
| (a la DrIFT)
| 
| 
| 
| --
| Best regards,
|  Bulat                            mailto:bulatz <at> HotPOP.com
| 
| 
| 
| _______________________________________________
| template-haskell mailing list
| template-haskell <at> haskell.org
| http://www.haskell.org/mailman/listinfo/template-haskell
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Bulat Ziganshin | 26 Oct 2005 09:48

Re[2]: RE: Documentation Template Haskell

Hello Tomasz,

Wednesday, October 26, 2005, 10:58:53 AM, you wrote:

TZ> I also use TH, mostly to generate instances of some classes and
TZ> for things like interpolation (this is the word they use in Perl-world
TZ> for this - I've used Perl several years ago) of variables in String literals.

can you publish these modules with examples of usage? one of TH
problems is severe lacking of light examples of its usage

TZ> There are probably many things that could  be done to make TH
TZ> more useful, and hopefully also more popular. Here are some I
TZ> thought about:
TZ> - allow to use TH to create rules for deriving instances of
TZ>   user defined type classes - so the end user doesn't have
TZ>   to know about TH - this is quite controversial, because on
TZ>   one hand it would make TH less visible, but on the other,
TZ>   library writers would be more willing to use it

i' m also think about this. smth like expansion of:

data T = ... deriving (GhcBinary)

to

data T = ...
$(deriveGhcBinary ''T)

--

-- 
Best regards,
 Bulat                            mailto:bulatz <at> HotPOP.com
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Gmane