showsPrec: cui bono?
A simple, primitive question: has anybody here used in a non-trivial way the showsPrec anti-parser? My students asked me what is it for, it is never used in the Hugs Prelude, OK, once: possible parentheses around fractions n%d. I explained that it is a good contraption to make one own pretty- printers, like readsPrec can be useful for simple, precedence-based parsers. But I am still unhappy. The associativity is not taken into account, and for non-trivial purposes both ...Prec functions seem not as useful as I would like. Any comments? Thanks. Jerzy Karczmarczuk ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: showsPrec: cui bono?
has anybody here used in a non-trivial way the showsPrec anti-parser?
Isn't the idea to make things trivial while avoiding performance
penalties? Perhaps: simple pretty-printing of abstract syntax trees?
I often use it to get simple debugging output for complex internal
data structures (first, use deriving; then, define showsPrec; if
that's still not good enough, do some real thinking..).
Anyway, this reminded me of a litte old hack of mine. Only trivial
use of showsPrec, but perhaps you'll like it anyway?-)
http://www.cs.ukc.ac.uk/people/staff/cr3/toolbox/haskell/R.hs
As with anything else in my toolbox, no warranty for nothing..
Cheers,
Claus
--- cut here
{-
Representative thingies..
A little hack to pair values with string representations
of their expressions. Useful if you want to explain what
map (+1) [1..4] or foldr1 (*) [1..5]
do, or if you want to demonstrate the difference between
foldr (+) 0 [1..4] and foldl (+) 0 [1..4]
Load this module into Hugs (Hugs mode) and type in some of
these examples to get an idea of what I mean. Also try
map (+) [1..4]
This could be extended in various directions, but I wanted to
keep things simple. I'm not convinced that extra complications
would be worth the effort.
Claus Reinke
-}
default (R Integer)
data R a = R {rep:: String
,val:: a
}
instance Show (R a) where
showsPrec _ a = showString (rep a)
instance Show (R a - R b) where
showsPrec _ f = showString (\\x-++(rep (f x)))
where
x = R{rep=x,val=error variable}
instance Show (R a - R b - R c) where
showsPrec _ f = showString (\\x y-++(rep (f x y)))
where
x = R{rep=x,val=error variable}
y = R{rep=y,val=error variable}
lift1 op a = R {rep=(++(rep op)++ ++(rep a)++)
,val= ( (val op) (val a) )
}
lift2 op a b = R {rep=(++(rep op)++ ++(rep a)++ ++(rep b)++)
,val= ( (val op) (val a) (val b) )
}
lift2infix op a b = R {rep=(++(rep a)++ ++(rep op)++ ++(rep b)++)
,val= ( (val a) `iop` (val b) )
}
where
iop = val op
instance (Num a,Show a) = Num (R a) where
(+)= lift2infix R{rep=+,val=(+)}
(-)= lift2infix R{rep=-,val=(-)}
(*)= lift2infix R{rep=*,val=(*)}
negate = lift1 R{rep=-,val=negate}
fromInteger a = (\fIa-R{rep=show fIa,val=fIa}) (fromInteger a)
instance (Eq a,Num a) = Eq (R a) where
a == b = (val a) == (val b)
instance (Ord a,Num a) = Ord (R a) where
a = b = (val a) = (val b)
instance (Enum a,Num a,Show a) = Enum (R a) where
fromEnum = fromEnum.val
toEnum a = R{rep=show a,val=toEnum a}
enumFrom x = map toEnum [fromEnum x..] -- missing in Hugs Prelude..
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Calling Haskell from Python / C++
Hi, I've just recently learned about Haskell, and I'm impressed by the abstractions and expressiveness that it affords. I'm particularly interested in it for a small parser project that I'm planning. However, my main programming languages are Python and C++, and for various reasons switching entirely to Haskell is completely out of the question (no flame-bait intended!). So here is what I envision: I write the main application in Python. I write a (hopefully) small Haskell module that: a) Calls back to the main Python app for reading the text to be parsed, preferably using laziness. b) Parses the text, and maybe processes a bit. c) Returns the parsed data-structure. (This may be tricky, but I think I know how to do it). This all seems quite cool. And it can also be very useful in a wide veriety of other circumstances, i.e. whenever a complex computation can be outsourced from Python/C++ to Haskell. But I can't find any way to interface Python and Haskell in this way. The FFI seems to allow for this, but there arn't any tools to actually do it... So my question is: How would one do such a thing? Thanks in advance, -JH __ Do you Yahoo!? U2 on LAUNCH - Exclusive greatest hits videos http://launch.yahoo.com/u2 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Haskell98 Report copyright
I hope we don't have a repeat of the MathWorld website shutdown.* I also can't find a webpage with the definition of Standard ML... only avaible in print from MIT Press? Chris * http://mathworld.wolfram.com/erics_commentary.html = Christopher Milton [EMAIL PROTECTED] __ Do you Yahoo!? U2 on LAUNCH - Exclusive greatest hits videos http://launch.yahoo.com/u2 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
1 line simple cat in Haskell
If you are steaming with compicated codes, then how about taking a break.
Let's play with a simple cat.
\begin{code}
main = mapM (=putChar) getCharS where getCharS = getChar:getCharS
\end{code}
Tested with ghc.
Works good except that you get some messages on stderror
because eof is not handled.
How would you suggest to neatly insert the error handling code into ?
P.S.
Instead of coding with C++,
I want to write my server main code like this.
server_main = mapM (=process.reply) where getReqS = getReq:getReqS
Only if I had enough time ... :-p
Using HDirect and so on ...
--
Ahn Ki-yung
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