TP | 24 Aug 11:36 2013
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reasons why Template Haskell does not propose something similar to Python exec() or eval()

Hi everybody,

I continue to learn and test Template Haskell (one more time thanks to John 
Lato for his post at:

http://www.mail-archive.com/haskell-cafe <at> haskell.org/msg106696.html

that made me understand a lot of things).

I have a question about the way Template Haskell is working. Why Template 
Haskell does not propose something similar to Python (or bash) exec() or 
eval(), i.e. does not offer the possibility to take a (quoted) string in 
input, to make abstract syntax in output (to be executed later in a splice 
$()).
For example, in Python, to make an affectation 'a="a"' programatically, I 
can simply do (at runtime; even if I am here only concerned with what 
Template Haskell could do, i.e. at compile time):
> def f(s): return '%s = \'%s\'' % (s,s)
> exec(f("a"))
> a
'a'

With Template Haskell, I am compelled to make a function returning the 
abstract syntax corresponding to variable declaration:

ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)

(see complete example in Post Scriptum).
This works fine, but it is less direct. I am sure that the Template Haskell 
approach has many advantages, but I am unable to list them. I think it is 
(Continue reading)

jean-christophe mincke | 24 Aug 12:35 2013
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Re: reasons why Template Haskell does not propose something similar to Python exec() or eval()

Hello,

Maybe you could have a look at Quasi Quotation.

Regards

J-C


On Sat, Aug 24, 2013 at 11:36 AM, TP <paratribulations <at> free.fr> wrote:
Hi everybody,

I continue to learn and test Template Haskell (one more time thanks to John
Lato for his post at:

http://www.mail-archive.com/haskell-cafe <at> haskell.org/msg106696.html

that made me understand a lot of things).

I have a question about the way Template Haskell is working. Why Template
Haskell does not propose something similar to Python (or bash) exec() or
eval(), i.e. does not offer the possibility to take a (quoted) string in
input, to make abstract syntax in output (to be executed later in a splice
$()).
For example, in Python, to make an affectation 'a="a"' programatically, I
can simply do (at runtime; even if I am here only concerned with what
Template Haskell could do, i.e. at compile time):
> def f(s): return '%s = \'%s\'' % (s,s)
> exec(f("a"))
> a
'a'

With Template Haskell, I am compelled to make a function returning the
abstract syntax corresponding to variable declaration:

ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)

(see complete example in Post Scriptum).
This works fine, but it is less direct. I am sure that the Template Haskell
approach has many advantages, but I am unable to list them. I think it is
important to have the reasons in mind. Could you help me?

Thanks in advance,

TP


PS: the complete Haskell example:

-----------------------------------
module MakeVard where
import Language.Haskell.TH

makeVard :: Monad m => String -> m [Dec]
-- Equivalent to "%s = \"%s\""
makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) []
]
-----------------------------------

tested by

-----------------------------------
{-# LANGUAGE TemplateHaskell #-}
import MakeVard

$(makeVard "a")

main = do

print a
-----------------------------------

resulting in
$ runghc -ddump-splices test.hs
test_makeVar.hs:1:1: Splicing declarations
    makeVard "a"
  ======>
    test_makeVar.hs:4:3-14
    a = "a"
"a"


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Tobias Dammers | 24 Aug 12:52 2013
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Re: reasons why Template Haskell does not propose something similar to Python exec() or eval()

IIRC you can use haskell-src-exts to parse a string into TH AST structures. There might even be a quasi-quoter for that; I don't have a real computer at hand right more, so you'll need to do some research of your own.

On Aug 24, 2013 11:37 AM, "TP" <paratribulations <at> free.fr> wrote:
Hi everybody,

I continue to learn and test Template Haskell (one more time thanks to John
Lato for his post at:

http://www.mail-archive.com/haskell-cafe <at> haskell.org/msg106696.html

that made me understand a lot of things).

I have a question about the way Template Haskell is working. Why Template
Haskell does not propose something similar to Python (or bash) exec() or
eval(), i.e. does not offer the possibility to take a (quoted) string in
input, to make abstract syntax in output (to be executed later in a splice
$()).
For example, in Python, to make an affectation 'a="a"' programatically, I
can simply do (at runtime; even if I am here only concerned with what
Template Haskell could do, i.e. at compile time):
> def f(s): return '%s = \'%s\'' % (s,s)
> exec(f("a"))
> a
'a'

With Template Haskell, I am compelled to make a function returning the
abstract syntax corresponding to variable declaration:

ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)

(see complete example in Post Scriptum).
This works fine, but it is less direct. I am sure that the Template Haskell
approach has many advantages, but I am unable to list them. I think it is
important to have the reasons in mind. Could you help me?

Thanks in advance,

TP


PS: the complete Haskell example:

-----------------------------------
module MakeVard where
import Language.Haskell.TH

makeVard :: Monad m => String -> m [Dec]
-- Equivalent to "%s = \"%s\""
makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) []
]
-----------------------------------

tested by

-----------------------------------
{-# LANGUAGE TemplateHaskell #-}
import MakeVard

$(makeVard "a")

main = do

print a
-----------------------------------

resulting in
$ runghc -ddump-splices test.hs
test_makeVar.hs:1:1: Splicing declarations
    makeVard "a"
  ======>
    test_makeVar.hs:4:3-14
    a = "a"
"a"


_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe <at> haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
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http://www.haskell.org/mailman/listinfo/haskell-cafe
TP | 24 Aug 23:45 2013
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Re: reasons why Template Haskell does not propose something similar to Python exec() or eval()

Tobias Dammers wrote:

> IIRC you can use haskell-src-exts to parse a string into TH AST
> structures. There might even be a quasi-quoter for that; I don't have a
> real computer at hand right more, so you'll need to do some research of
> your own.

Thanks Tobias, it led me to the right path. There is indeed a solution in 
Language.Haskell.Meta:

$ ghci
> :m Language.Haskell.Meta
> parseDecs "s=s+1"
Right [ValD (VarP s) (NormalB (UInfixE (VarE s) (VarE +) (LitE (IntegerL 
1)))) []]
> parseExp "x+1"
Right (UInfixE (VarE x) (VarE +) (LitE (IntegerL 1)))
> :i parseDecs
parseDecs ::
  String -> Either String [Language.Haskell.TH.Syntax.Dec]
        -- Defined in `Language.Haskell.Meta.Parse'
> :i parseExp 
parseExp :: String -> Either String Language.Haskell.TH.Syntax.Exp
        -- Defined in `Language.Haskell.Meta.Parse'

Thanks,

TP
Marc Weber | 24 Aug 13:12 2013
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Picon

Re: reasons why Template Haskell does not propose something similar to Python exec() or eval()

Excerpts from TP's message of Sat Aug 24 11:36:04 +0200 2013:
> Haskell does not propose something similar to Python (or bash) exec() or 
> eval(), i.e. does not offer the possibility to take a (quoted) string in 

You actually have eval/exec like features. You can run "ghc" modules in a
haskell application to compile a module, then run that code.

Eg see this example to get started
http://mawercer.de/tmp/haskell-dyn-loading-example.zip

Origin of most ideas was this site:
http://codeutopia.net/blog/2011/08/20/adventures-in-haskell-dynamic-loading-and-compiling-of-modules/

Marc Weber
Dag Odenhall | 25 Aug 11:42 2013
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Re: reasons why Template Haskell does not propose something similar to Python exec() or eval()

There's a proposal for adding a proper Haskell QuasiQuoter as part of template-haskell. Until then, as others have noted your best option is the haskell-src-meta package, but be aware that this uses a separate parser.



On Sat, Aug 24, 2013 at 11:36 AM, TP <paratribulations <at> free.fr> wrote:
Hi everybody,

I continue to learn and test Template Haskell (one more time thanks to John
Lato for his post at:

http://www.mail-archive.com/haskell-cafe <at> haskell.org/msg106696.html

that made me understand a lot of things).

I have a question about the way Template Haskell is working. Why Template
Haskell does not propose something similar to Python (or bash) exec() or
eval(), i.e. does not offer the possibility to take a (quoted) string in
input, to make abstract syntax in output (to be executed later in a splice
$()).
For example, in Python, to make an affectation 'a="a"' programatically, I
can simply do (at runtime; even if I am here only concerned with what
Template Haskell could do, i.e. at compile time):
> def f(s): return '%s = \'%s\'' % (s,s)
> exec(f("a"))
> a
'a'

With Template Haskell, I am compelled to make a function returning the
abstract syntax corresponding to variable declaration:

ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)

(see complete example in Post Scriptum).
This works fine, but it is less direct. I am sure that the Template Haskell
approach has many advantages, but I am unable to list them. I think it is
important to have the reasons in mind. Could you help me?

Thanks in advance,

TP


PS: the complete Haskell example:

-----------------------------------
module MakeVard where
import Language.Haskell.TH

makeVard :: Monad m => String -> m [Dec]
-- Equivalent to "%s = \"%s\""
makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) []
]
-----------------------------------

tested by

-----------------------------------
{-# LANGUAGE TemplateHaskell #-}
import MakeVard

$(makeVard "a")

main = do

print a
-----------------------------------

resulting in
$ runghc -ddump-splices test.hs
test_makeVar.hs:1:1: Splicing declarations
    makeVard "a"
  ======>
    test_makeVar.hs:4:3-14
    a = "a"
"a"


_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe <at> haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe <at> haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Gmane