Re: [Haskell-cafe] 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] reasons why Template Haskell does not propose something similar to Python exec() or eval()
There's a
proposalhttp://ghc.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal#PartD:quasiquotationfor
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 paratribulati...@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@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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Re: [Haskell-cafe] reasons why Template Haskell does not propose something similar to Python exec() or eval()
Hello,
Maybe you could have a look at Quasi
Quotationhttp://www.haskell.org/haskellwiki/Quasiquotation
.
Regards
J-C
On Sat, Aug 24, 2013 at 11:36 AM, TP paratribulati...@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@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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Re: [Haskell-cafe] 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 paratribulati...@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@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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Re: [Haskell-cafe] 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
