I supposed the simple solution might be CPP: #define defObj(NAME) newtype NAME = NAME Obj deriving (A,B,C,D) and then use defObj (MyType) I have heard some people, however, say that CPP macros are horrible in Haskell, so is there a better solution? Kevin On Sep 14, 10:34 am, Miguel Mitrofanov wrote:

  Sorry, got stupid today. Won't help.

14.09.2010 12:29, Miguel Mitrofanov пишет:

...

 class (A x, B x, C x, D x) => U x

...

?

...

14.09.2010 12:24, Kevin Jardine пишет:

...

I have a set of wrapper newtypes that are always of the same format:

...

...

newtype MyType = MyType Obj deriving (A,B,C,D)

...

...

where Obj, A, B, C, and D are always the same. Only MyType varies.

...

...

A, B, C, and D are automagically derived by GHC using the

...

...

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

...

...

feature.

...

...

I would like to use some macro system (perhaps Template Haskell?) to reduce this to something like

...

...

defObj MyType

...

...

I've read through some Template Haskell documentation and examples, but I find it intimidatingly hard to follow. Does anyone has some code suggestions or pointers to something similar?

...

...

Alternatively, is there any way in standard Haskell to define some kind of union class:

...

...

U = (A, B, C, D)

...

...

and then using

...

...

newtype MyType = MyType Obj deriving U

...

...

which would at least be shorter? _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Hmm - It seems to work if the code is defined before my main function and not after it. Does this have to do with TH being part of the compile process and so the order matters? Kevin On Sep 14, 6:03 pm, Kevin Jardine wrote:

Thanks Serguey!

The library code compiles, but when I try to use it in client code:

a. I get:

Not in scope: type constructor or class 'A'

and even stranger,

b. GHC cannot find any of my code after the

$(mkNewType "A")

and claims that all the functions I defined there are also not in scope.

Any ideas?

The CPP solution works but Template Haskell is definitely cooler, so it would be great to get this to work!

Kevin

On Sep 14, 2:29 pm,  Zefirov wrote:

...

2010/9/14 Kevin Jardine :

...

...

I would like to use some macro system (perhaps Template Haskell?) to reduce this to something like

...

...

defObj MyType

...

...

I've read through some Template Haskell documentation and examples, but I find it intimidatingly hard to follow. Does anyone has some code suggestions or pointers to something similar?

...

The solutions first: ------------------------------------------------- {-# LANGUAGE TemplateHaskell #-}

...

module T(mkNewType) where

...

import Language.Haskell.TH

...

decls = [d|newtype TempDecl = TempDecl Int deriving (Eq,Ord,Show)|] decl = do         [d] <- decls         runIO $ print d -- just to show inetrnals         return d

...

mkNewType :: String -> Q [Dec] mkNewType n = do         d <- decl         let name = mkName n         return $ (\x -> [x]) $ case d of                 (NewtypeD cxt _ argvars (NormalC _ args) derivings) ->                         NewtypeD cxt name argvars (NormalC name args) derivings -------------------------------------- I took perfectly valid declaration, dissected it using case analysis and changed relevant parts.

...

And an example client: ------------------------------------- {-# LANGUAGE TemplateHaskell #-}

...

import T

...

$(mkNewType "A") ------------------------------------- It all work together.

...

I studied how to use Template Haskell that way: I obtained declarations of what I need, printed them and looked through documentation for relevant data types and constructors. It's not harder that any other library in Haskell, actually. _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

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OK, thanks for everyone's help! Serguey's code works very well now. Kevin On Sep 14, 6:14 pm, Erik Hesselink wrote:

Yes, if you use template haskell, all top level functions and values have to be defined before you use them.

Erik

On Tue, Sep 14, 2010 at 18:11, Kevin Jardine wrote:

...

Hmm - It seems to work if the code is defined before my main function and not after it.

...

Does this have to do with TH being part of the compile process and so the order matters?

...

Kevin

...

On Sep 14, 6:03 pm, Kevin Jardine wrote:

...

Thanks Serguey!

...

...

The library code compiles, but when I try to use it in client code:

...

...

a. I get:

...

...

Not in scope: type constructor or class 'A'

...

...

and even stranger,

...

...

b. GHC cannot find any of my code after the

...

...

$(mkNewType "A")

...

...

and claims that all the functions I defined there are also not in scope.

...

...

Any ideas?

...

...

The CPP solution works but Template Haskell is definitely cooler, so it would be great to get this to work!

...

...

Kevin

...

...

On Sep 14, 2:29 pm,  Zefirov wrote:

...

...

...

2010/9/14 Kevin Jardine :

...

...

...

...

I would like to use some macro system (perhaps Template Haskell?) to reduce this to something like

...

...

...

...

defObj MyType

...

...

...

...

I've read through some Template Haskell documentation and examples, but I find it intimidatingly hard to follow. Does anyone has some code suggestions or pointers to something similar?

...

...

...

The solutions first: ------------------------------------------------- {-# LANGUAGE TemplateHaskell #-}

...

...

...

module T(mkNewType) where

...

...

...

import Language.Haskell.TH

...

...

...

decls = [d|newtype TempDecl = TempDecl Int deriving (Eq,Ord,Show)|] decl = do         [d] <- decls         runIO $ print d -- just to show inetrnals         return d

...

...

...

mkNewType :: String -> Q [Dec] mkNewType n = do         d <- decl         let name = mkName n         return $ (\x -> [x]) $ case d of                 (NewtypeD cxt _ argvars (NormalC _ args) derivings) ->                         NewtypeD cxt name argvars (NormalC name args) derivings -------------------------------------- I took perfectly valid declaration, dissected it using case analysis and changed relevant parts.

...

...

...

And an example client: ------------------------------------- {-# LANGUAGE TemplateHaskell #-}

...

...

...

import T

...

...

...

$(mkNewType "A") ------------------------------------- It all work together.

...

...

...

I studied how to use Template Haskell that way: I obtained declarations of what I need, printed them and looked through documentation for relevant data types and constructors. It's not harder that any other library in Haskell, actually. _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

...

...

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Hi John, That's what I had originally. However, some people have made critical comments about CPP macros on this list and I thought that TH was considered the better option. What do other people think? Serguey's code is great in any case as it gives me a clearer understanding on how TH works. Kevin On Sep 14, 11:01 pm, John Meacham wrote:

On Tue, Sep 14, 2010 at 01:24:16AM -0700, Kevin Jardine wrote:

...

I have a set of wrapper newtypes that are always of the same format:

...

newtype MyType = MyType Obj deriving (A,B,C,D)

...

where Obj, A, B, C, and D are always the same. Only MyType varies.

...

A, B, C, and D are automagically derived by GHC using the

...

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

...

feature.

...

I would like to use some macro system (perhaps Template Haskell?) to reduce this to something like

...

defObj MyType

How about the straightforward?

...

{-# LANGUAGE CPP #-} #define defObj(t)   newtype t = t Obj deriving (A,B,C,D)

...

defObj(Foo) defObj(Bar) ....

It has the advantage of being (de facto) portable.

        John

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Hi Malcolm, In this case, I am counting on GHC's {-# LANGUAGE GeneralizedNewtypeDeriving #-} feature to derive the instances for the classes I am including in the deriving clause. So perhaps portability is not a big issue here in any case. I do think that defObj(MyType) looks a bit cleaner than $(defObj "MyType") so I am starting to lean back towards the CPP solution after all. CPP is not always the best option, but perhaps it is in this case. Kevin On Sep 15, 10:31 am, Malcolm Wallace wrote:

...

That's what I had originally. However, some people have made critical comments about CPP macros on this list and I thought that TH was considered the better option.

I was one of those people advising against the use of CPP macros.   However, Template Haskell is ghc-only, and is unlikely ever to be   implemented by any other Haskell compiler.  Thus CPP, for all its   faults, may be the better solution here, simply because it is portable.

(I also note in passing that ghc's core libraries themselves use   exactly this kind of CPP macro to generate lots of tedious boilerplate.)

Regards,      Malcolm

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Hi Ben, Good point! I can confirm that it does compile under GHC 6.12. So really the same number of characters either way. Kevin On Sep 15, 4:49 pm, Ben Millwood wrote:

On Wed, Sep 15, 2010 at 2:11 PM, Kevin Jardine wrote:

...

I do think that

...

defObj(MyType)

...

looks a bit cleaner than

...

$(defObj "MyType")

I believe as of GHC 6.12 you no longer need the $() around top-level splices. So that would just be:

defObj "MyType" _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

On 9/15/10 1:31 AM, Malcolm Wallace wrote:

[...] However, Template Haskell is ghc-only, and is unlikely ever to be implemented by any other Haskell compiler. [...]

Could it be implemented as a separate preprocessor? Cheers, Greg

#define defObj(t) newtype t = t Obj deriving (A,B,C,D)

Blasphemy! :) On 14 September 2010 23:01, John Meacham wrote:

On Tue, Sep 14, 2010 at 01:24:16AM -0700, Kevin Jardine wrote:

...

I have a set of wrapper newtypes that are always of the same format:

newtype MyType = MyType Obj deriving (A,B,C,D)

where Obj, A, B, C, and D are always the same. Only MyType varies.

A, B, C, and D are automagically derived by GHC using the

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

feature.

I would like to use some macro system (perhaps Template Haskell?) to reduce this to something like

defObj MyType

How about the straightforward?

...

{-# LANGUAGE CPP #-} #define defObj(t) newtype t = t Obj deriving (A,B,C,D)

defObj(Foo) defObj(Bar) ....

It has the advantage of being (de facto) portable.

John

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