Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
On Sat, 28 Jan 2006, Andrew Savige wrote: Haskell beginner using GHC. I have a function to do some simple arithmetic at run time: myeval :: Int -> Int -> String -> Int myeval x y "+" = (+) x y myeval x y "-" = (-) x y myeval x y "*" = (*) x y -- ... Additionally to the other suggestions, I advise using a custom type data Op = Add | Sub | Mul instead of String, if possible. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
On 1/28/06, Andrew Savige <[EMAIL PROTECTED]> wrote:
> --- Brian Hulley wrote:
> >> def myeval(x, y, op)
> >> x.send op, y
> >> end
> >
> > This could be done in Haskell by
> >
> > myeval :: a->b->(a -> b -> c) -> c
> > myeval x y op = op x y
> >
> > eg myeval (+) 1 2
>
> Though the following program does indeed work:
>
> myeval :: (Int -> Int -> Int) -> Int -> Int -> Int
> myeval op x y = op x y
>
> main :: IO ()
> main = do putStrLn $ show (myeval (+) 2 3)
>
> I can't hardwire (+) because the operator is unknown at
> compile time; it is input to the program at run time as
> a string ("+" in this example).
>
> My problem is how to convert the string "+" to the operator
> (+) at run time. Well, I could do it with a lookup table:
>
> mycvt :: String -> (Int -> Int -> Int)
> mycvt "+" = (+)
> mycvt "-" = (-)
> mycvt "*" = (*)
>
> but that is what I'm trying to avoid (I was naively hoping
> that the read function could magically do it somehow :-).
>
It's probably a bit more heavy-weight than what you were looking for,
but take a look at:
http://www.cse.unsw.edu.au/~dons/hs-plugins/
It allows you to load and execute Haskell code at run-time. I suppose
this should also be possible by importing some part of GHC (perhaps
GHCi).
/S
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Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
--- Brian Hulley wrote:
>> def myeval(x, y, op)
>> x.send op, y
>> end
>
> This could be done in Haskell by
>
> myeval :: a->b->(a -> b -> c) -> c
> myeval x y op = op x y
>
> eg myeval (+) 1 2
Though the following program does indeed work:
myeval :: (Int -> Int -> Int) -> Int -> Int -> Int
myeval op x y = op x y
main :: IO ()
main = do putStrLn $ show (myeval (+) 2 3)
I can't hardwire (+) because the operator is unknown at
compile time; it is input to the program at run time as
a string ("+" in this example).
My problem is how to convert the string "+" to the operator
(+) at run time. Well, I could do it with a lookup table:
mycvt :: String -> (Int -> Int -> Int)
mycvt "+" = (+)
mycvt "-" = (-)
mycvt "*" = (*)
but that is what I'm trying to avoid (I was naively hoping
that the read function could magically do it somehow :-).
/-\
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Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
Brian Hulley wrote: eg myeval (+) 1 2 myeval 1 2 (+)-- I *always* seem to make at least one mistake per post ;-) (I originally wrote the code to take the op first, which is the usual Haskell convention so that you can do useful things with (myeval someop) but then I noticed you had the op arg last in your Ruby example so I changed the code but forgot to change my example...) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
Andrew Savige wrote: --- Cale Gibbard wrote: Apart from moving to a lookup Map or something, a simple reordering of the arguments allows you to shorten things up a bit: myeval :: String -> Int -> Int -> Int myeval "+" = (+) myeval "-" = (-) myeval "*" = (*) etc. Thanks to all for the excellent suggestions. I'm liking the Haskell version of this function a lot more now. :-) To help me learn Haskell, I'm converting a small Ruby program to Haskell. In Ruby this can be done in one line: def myeval(x, y, op) x.send op, y end This could be done in Haskell by myeval :: a->b->(a -> b -> c) -> c myeval x y op = op x y eg myeval (+) 1 2 Regards, Brian. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
--- Cale Gibbard wrote: > Apart from moving to a lookup Map or something, a simple reordering > of the arguments allows you to shorten things up a bit: > > myeval :: String -> Int -> Int -> Int > myeval "+" = (+) > myeval "-" = (-) > myeval "*" = (*) > etc. Thanks to all for the excellent suggestions. I'm liking the Haskell version of this function a lot more now. :-) To help me learn Haskell, I'm converting a small Ruby program to Haskell. In Ruby this can be done in one line: def myeval(x, y, op) x.send op, y end I had a feeling this sort of dynamic sending of messages to objects at run time was impossible in Haskell, hence my question. What I'm still unsure about is why this sort of thing is impossible in Haskell. Is it a fair comment to state that this sort of thing is impossible in Haskell as a consequence of its static typing? Or could it be done in a static typed language with more run time support? Despite being longer, overall I prefer the Haskell version because it is faster and "safer" (in that a number of run time errors in the Ruby version are caught at compile time in the Haskell version). /-\ Do you Yahoo!? Find a local business fast with Yahoo! Local Search http://au.local.yahoo.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
On 28/01/06, Andrew Savige <[EMAIL PROTECTED]> wrote: > Haskell beginner using GHC. > > I have a function to do some simple arithmetic at run time: > > myeval :: Int -> Int -> String -> Int > myeval x y "+" = (+) x y > myeval x y "-" = (-) x y > myeval x y "*" = (*) x y > -- ... > > While that works, I'm curious to know if it can be done more > elegantly. I'm thinking of something like: > > myeval :: Int -> Int -> String -> Int > myeval x y op = (read op) x y > > Thanks, > /-\ Apart from moving to a lookup Map or something, a simple reordering of the arguments allows you to shorten things up a bit: myeval :: String -> Int -> Int -> Int myeval "+" = (+) myeval "-" = (-) myeval "*" = (*) etc. - Cale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
BTW: There isn't an easy way to magically derive opTable. We're relying
on the fact that these simple functions all have the same type.
Ben Lippmeier wrote:
opTable
= [ ("+", (+))
, ("-", (-)) ... ]
myeval x y op
= letJust fun = lookup op opTable
in x `fun` y
You could also do
myeval x y op
= case op of
"+" -> x + y
"-" -> x - y
...
is probably the "smallest" way.
Ben.
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Re: [Haskell-cafe] Evaluating arithmetic expressions at run time
Andrew Savige wrote:
> Haskell beginner using GHC.
Hello there!
How about,
opTable
= [ ("+", (+))
, ("-", (-)) ... ]
myeval x y op
= let Just fun = lookup op opTable
in x `fun` y
?
I have a function to do some simple arithmetic at run time:
myeval :: Int -> Int -> String -> Int
myeval x y "+" = (+) x y
myeval x y "-" = (-) x y
myeval x y "*" = (*) x y
-- ...
While that works, I'm curious to know if it can be done more
elegantly. I'm thinking of something like:
myeval :: Int -> Int -> String -> Int
myeval x y op = (read op) x y
Thanks,
/-\
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