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

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

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

jean-christophe mincke
Hello,

Maybe you could have a look at Quasi Quotation.

Regards

J-C


On Sat, Aug 24, 2013 at 11:36 AM, TP <[hidden email]> 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@.../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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Haskell-Cafe mailing list
[hidden email]
http://www.haskell.org/mailman/listinfo/haskell-cafe


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

Tobias Dammers
In reply to this post by TP

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" <[hidden email]> 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@.../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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[hidden email]
http://www.haskell.org/mailman/listinfo/haskell-cafe

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

Marc Weber
In reply to this post by TP
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

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

TP
In reply to this post by Tobias Dammers
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


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

Dag Odenhall
In reply to this post by TP

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 <[hidden email]> 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@.../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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[hidden email]
http://www.haskell.org/mailman/listinfo/haskell-cafe


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