sorry for the empty post before - what i wanted to says is, but the send button was to quickly reached: --------------------- i think this is a perfect example that the statement made earlier, namely that list overloading confuses beginners, but overloading numbers does not, is wrong. i would therefore recommend (1) that all operators in Haskell' are made into classes to allow their overloading - in accordance with the polymorphic character of haskell (today). and (2) that the number system with the default conversions is critically evaluated. in my programming, the overloading of numbers and the defaults come in may way more often than not (i'd rather not have it automatically done! like many other things in haskell which are not automatic). overloading of numbers - as currently done - makes it difficult to use GHCi as a convenient and easy to use desktop calculator - more flexible and powerful than most others and in competition with mathematica and mathcad etc. (there is a large user pool!) andrew On 6 Sep 07 12 22, Andrew U. Frank wrote:
On 5 Sep 07 23 28, Brandon S. Allbery KF8NH wrote:
On Sep 5, 2007, at 21:10 , Tomi Owens wrote:
Prelude> let f (a,b) = a * floor (100000/b) Prelude> f(2,5) 40000
This function works just as I want it to.
Now I try creating a list:
Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)]
and this works So now I try to apply the function to the list:
Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a]
and I get this result:
<interactive>:1:5: Ambiguous type variable `t' in the constraints: `Integral t' arising from use of `f' at <interactive>:1:5 `RealFrac t' arising from use of `f' at <interactive>:1:5 Probable fix: add a type signature that fixes these type variable
(s)
I'm sorry, but I don't quite get how to set the type signature
and
how it will apply to my function...
The problem here is that (assuming the a\sup{2} etc. are actually a^2) the (^) operator expects and returns Integrals, but (/) requires
a RealFrac. Thus, the type of your list comprehension is inferred to
be [(Integer,Integer)] but needs to be RealFrac a => [(Integer,a)] (or, more simply, [(Integer,Double)].
Prelude> let f (a,b) = a * floor (100000/b) Prelude> :t f f :: (RealFrac t1, Integral t) => (t, t1) -> t Prelude> let v :: [(Integer,Double)]; v = [(a^2 + b^2,fromIntegral
a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t v v :: [(Integer, Double)] Prelude> map f v [200000,250000,400000,333330,433329,599994,425000]
-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
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