Here is my code:
n = [(a,b) | a <- [1..5],b <- [1..5]]
calcBmis xs = [bmi | (w, h) <- xs,let bmi = w / h ^ 2]
When trying to apply calcBmis to n, I get the following error:
*Charana> calcBmis n
<interactive>:220:1:
No instance for (Fractional Integer)
arising from a use of ‘calcBmis’
In the expression: calcBmis n
In an equation for ‘it’: it = calcBmis n
Further investigation in ghci:
*Charana> :t calcBmis
calcBmis :: Fractional t => [(t, t)] -> [t]
*Charana> :t n
n :: [(Integer, Integer)]
What I'm assuming is that the list I produce is of type (Integer,Integer), but that cannot be processed in the calcBmis, which only takes in Fractional. Any idea how to fix this problem?
you can use div instead of (/):
calcBmis xs = [ bmi | (w,h) <- xs, let bmi = (w `div` h)^2 ]
Prelude> :t calcBmis
calcBmis :: Integral t => [(t, t)] -> [t]
Prelude> calcBmis n
[1,0,0,0,0,4,1,0,0,0,9,1,1,0,0,16,4,1,1,0,25,4,1,1,1]
as you can see this version can deal with all Integral values - but of course will truncate (because of div).
or you can map everything with fromIntegral:
calcBmis xs = [ bmi | (w,h) <- xs, let bmi = (fromIntegral w / fromIntegral h)^2 ]
Prelude> :t calcBmis
calcBmis:: (Fractional t, Integral t1, Integral t2) => [(t1, t2)] -> [t]
Which then will produce fractional values:
Prelude> calcBmis n
[1.0,0.25,0.1111111111111111
,6.25e-2
,4.000000000000001e-2
,4.0
,1.0
,0.4444444444444444
, ... ]
in either case it will work with all inputs as long as they are an instance of Integral - the second version will even accept pairs of different integrals ;)