[Haskell-cafe] Re: [Haskell] (no subject)

Wed Sep 5 18:29:58 EDT 2007

I think you want something like this

{-# OPTIONS -fglasgow-exts #-}

f :: (Integer, Float) -> Integer
f (a,b) = a * floor (100000/b) 

lst :: [(Integer, Integer)]
lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= 
a]

lst3 = map (f) ( map ( intTupToFloatTup  ) lst )

intTupToFloatTup :: (Integer, Integer) -> (Integer, Float)
intTupToFloatTup (int1, int2) = (int1, fromInteger int2)

load the whole thing into ghci with ghci proggie.hs

when I have this type of problem, my usual approach is to put the code 
into a text file, load that in ghci, derive type sigs on the functions 
that work, and then see if I can figure out
the mismatch.

you could probably get a fast answer to this kind of question on the 
#haskell irc channel as well.

hope this helps,

thomas.

"Scott Williams" <williasr at google.com> 
Sent by: haskell-cafe-bounces at haskell.org
09/05/2007 05:28 PM

To
"Tomi Owens" <t.owens at hautlieu.sch.je>
cc
haskell-cafe at haskell.org
Subject
[Haskell-cafe] Re: [Haskell] (no subject)

[bcc haskell, cc haskell-cafe]

On 9/5/07, Tomi Owens <t.owens at hautlieu.sch.je> wrote:
Hi there. I'm a teacher of Maths and am working my way through the Euler 
Project problems for fun. I have mostly been using Basic, but have read up 
about Haskell and think it looks like a sensible way to solve many of the 
problems. 

OK, so I've downloaded GHCi and am trying to teach myself. 

So far I have done this: 

  ___         ___ _ 
 / _ \ /\  /\/ __(_) 
/ /_\// /_/ / /  | |      GHC Interactive, version 6.6.1, for Haskell 98. 
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/ 
\____/\/ /_/\____/|_|      Type :? for help. 

Loading package base ... linking ... done. 
Prelude> let f (a,b) = a * floor (100000/b) 
Prelude> f(2,5) 
40000 

Here you can find out type ghci has inferred for this function.
> :t f
f :: (RealFrac b, Integral b1) => (b1, b) -> b1

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)] 

Let's assign this to an intermediate variable so we can query it's type:

Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + 
b^2 < 20, b <= a]
Prelude> lst 
[(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)]
Prelude> :t lst
lst :: [(Integer, Integer)]

aha; here's the source of the type mismatch:
Prelude> :t floor
floor :: (RealFrac a, Integral b) => a -> b 

Floor has to take a RealFrac. According to hoogle[1], we can use various 
floating-point approximations (Float, Double, CFloat, etc) or we can use 
the exact Rational type.
[1] http://haskell.org/hoogle/?q=RealFrac

You can get your types to match by declaring your list to be of type 
[(Rational, Rational)] either by explicitly typing one of the untyped 
variables or the entire expression: 
Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <- 
[1..4], a^2 + b^2 < 20, b <= a]
Prelude> :t lst
lst :: [(Rational, Rational)]
Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <- 
[1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] 
Prelude> :t lst
lst :: [(Rational, Rational)]

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... 

Thanks, 

Hope this helps

Tomi 
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