Re: [Haskell-cafe] definition of the term combinator
Thanks. I found the explanation given at the link quite useful in shedding the confusion I had had. Thanks and regards, -Damodar Kulkarni On Sat, Aug 24, 2013 at 10:57 AM, Jason Dagit dag...@gmail.com wrote: On Fri, Aug 23, 2013 at 9:09 PM, damodar kulkarni kdamodar2...@gmail.comwrote: Hello, The word combinator is used several times in the Haskell community. e.g. parser combinator, combinator library etc. Is it exactly the same term that is used in the combinatory logic ? A combinator is a higher-order function that uses *only function application* and earlier defined combinators to define a result from its arguments. [1] It seems, the term combinator as in, say, parser combinator, doesn't have much to do with the *only function application* requirement of the combinatory logic, per se. If the above observation holds, is the term combinator as used in the Haskell community, properly defined? In other words: Where can I find a formal and precise definition of the term combinator, as a term used by the Haskell community to describe something? Good question. I believe this article addresses the points you raise: http://www.haskell.org/haskellwiki/Combinator ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] 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@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
Re: [Haskell-cafe] GHC flags: optghc
That's not a GHC flag; it's a haddock flag. Haddock (which, in case you're not familiar with it, is a program to generate documentation from Haskell source code) uses GHC, and the `optghc` flag lets you pass options to GHC when you invoke Haddock. See [the Haddock docs of the 6.12 era][1], on page 3. It's also entirely possible that some program besides Haddock uses a flag of the same name (for the same purpose, one would hope). [1]: http://www.haskell.org/ghc/docs/6.12.3/haddock.pdf 2013/8/23 jabolo...@google.com Hi, I am using GHC version 6.12.1. What is optghc ? I can't find that information anywhere... Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] TypeLits Typeable
Hello Cafe,
I was playing around with TypeLits in combination with Typeable (using
GHC 7.7.7.20130812 FWIW), but was surprised to find Symbols aren't
Typeable, and as such the following doesn't work. Is this intentional,
or am I missing something?
Thanks,
Nicolas
{-# LANGUAGE DataKinds,
KindSignatures,
DeriveFunctor,
DeriveDataTypeable #-}
module Main where
import Data.Typeable
import GHC.TypeLits
data NoSymbol n a b = NoSymbol a b
deriving (Typeable)
data WithSymbol (n :: Symbol) a b = WithSymbol a b
deriving (Typeable)
data Sym
deriving (Typeable)
main :: IO ()
main = do
print $ typeOf (undefined :: NoSymbol Sym Int Int)
let d = undefined :: WithSymbol sym Int Int
{-
print $ typeOf d
No instance for (Typeable Symbol sym)
arising from a use of ‛typeOf’
-}
return ()
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Re: [Haskell-cafe] Yet Another Forkable Class
Hi. On 23 August 2013 13:29, Nicolas Trangez nico...@incubaid.com wrote: Did anyone ever consider using type-level literals (strings) to 'name' effects (or transformer layers when using monad stacks)? Edwin Brady had this in his effects library in Idris. http://www.idris-lang.org/documentation/effects/ Ozgur. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Template Haskell: let statement in a splice put in the main = do part of a program?
Hi,
I continue to test Template Haskell, and I have some difficulties to use a
splice $() in a do contained in the main part of a program. Here is an
example. I want to make a splice that does `let a=a` in my code.
$ cat MakeLetStatement.hs
{-# LANGUAGE TemplateHaskell #-}
module MakeLetStatement where
import Language.Haskell.TH
makeLetStatement :: String - ExpQ
makeLetStatement s = return $ DoE $ [ LetS $ [ ValD (VarP $ mkName s)
(NormalB $ LitE $ StringL s) [] ]]
$ cat test_MakeLetStatement.hs
{-# LANGUAGE TemplateHaskell #-}
import MakeLetStatement
main = do
$(makeLetStatement a)
-- print a
Note I have commented print a because otherwise I obtain Not in scope:
`a' that shows that `a` has not been defined correctly, but does not show
whether my splice has been correctly expanded (I use --dump-splices GHC
option, but it seems it is not working for splices in the main = do part).
I obtain:
$ runghc -ddump-splices test_MakeLetStatement.hs
test_MakeLetStatement.hs:7:3:
Illegal last statement of a 'do' block:
let a = a
(It should be an expression.)
When splicing a TH expression: do let a = a
In a stmt of a 'do' block: $(makeLetStatement a)
In the expression: do { $(makeLetStatement a) }
In an equation for `main': main = do { $(makeLetStatement a) }
That shows that my splice has been correctly expanded: we have `let a =
a`. However, what happens is the same as in the following dummy script, we
have in fact defined a do inside the first do (with DoE), and so we
obtain an error because the last statement in a do block should be an
expression.
main = do
do let a = a
print a
So my code does not work, without surprise, but in fact my problem is to
transform a LetS statement:
http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.0.0/Language-Haskell-TH.html#v:LetS
that has type Stmt, in an ExpQ that seems to be the only thing that we can
put in a splice. I have found that it can only be done by doE (or DoE) and
compE (or CompE) according to
http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.0.0/Language-Haskell-TH.html#v:doE
But doE is not a solution as we have seen above, and compE is to construct
list comprehensions, which is a different thing.
So, is there any solution to my problem?
Thanks in advance,
TP
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Re: [Haskell-cafe] Template Haskell: let statement in a splice put in the main = do part of a program?
On Sat, Aug 24, 2013 at 11:00 AM, TP paratribulati...@free.fr wrote:
main = do
$(makeLetStatement a)
-- print a
Is that the actual indentation you used? Because it's wrong if so, and the
error you would get is the one you're reporting. Indentation matters in
Haskell.
In an equation for `main': main = do { $(makeLetStatement a) }
You cannot *end* a do with a let-statement; it requires something else
following it. You have nothing following it, as shown by the above fragment
from the error message.
--
brandon s allbery kf8nh sine nomine associates
allber...@gmail.com ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net
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Re: [Haskell-cafe] Template Haskell: let statement in a splice put in the main = do part of a program?
Brandon Allbery wrote:
main = do
$(makeLetStatement a)
-- print a
Is that the actual indentation you used? Because it's wrong if so, and the
error you would get is the one you're reporting. Indentation matters in
Haskell.
Yes, it matters, but not after main = do: all the lines can start at the
beginning of the line. Am I wrong? Or do I not understand what you say?
In an equation for `main': main = do { $(makeLetStatement a) }
You cannot *end* a do with a let-statement; it requires something else
following it. You have nothing following it, as shown by the above
fragment from the error message.
Yes, I have explained why: to be able to see the evaluation of the splice;
otherwise I obtain Not in scope: `a' if I uncomment -- print a at the
end of my code; I have explained everything in my initial post.
TP
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