Re: [Haskell-cafe] Template Haskell: let statement in a splice put in the main = do part of a program?
adam vogt wrote:
TH quotes limited as you've noticed. One way to generate similar code
is to note that:
do
let x = y
z
is the same as let x = y in do z. You can generate the latter with
something like the following file, but the `a' isn't in scope for the
second argument to makeLetStatement. The uglier $(dyn a) works,
though I suppose it's more verbose than manually in-lining the
variable a.
{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
main = $(let
makeLetStatement :: String - ExpQ - ExpQ
makeLetStatement s rest = letE [ valD (varP (mkName s))
(normalB $ stringE s) []]
rest
in makeLetStatement a [| print $(dyn a) |] )
Thanks Adam.
Unfortunately, this solution is not satisfying because the goal is to put
only one mention to a in the main part, putting all the repetitive code
and ExpQ's in a separate module. Tonight, I've tried hard one more time
without more success.
Maybe I have to stick to non-let expressions in the main part of a script,
when it comes to TH. It should nevertheless allow me to call functions, make
tests, etc.
Thanks,
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:
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?
Hi TP,
TH quotes limited as you've noticed. One way to generate similar code
is to note that:
do
let x = y
z
is the same as let x = y in do z. You can generate the latter with
something like the following file, but the `a' isn't in scope for the
second argument to makeLetStatement. The uglier $(dyn a) works,
though I suppose it's more verbose than manually in-lining the
variable a.
{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
main = $(let
makeLetStatement :: String - ExpQ - ExpQ
makeLetStatement s rest = letE [ valD (varP (mkName s))
(normalB $ stringE s) []]
rest
in makeLetStatement a [| print $(dyn a) |] )
--
Adam
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[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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