It is because (**) takes floating point numbers. In your first equation
both 4s are different. The second 4 is an Integer but the first is a some
Floating type aka 4.0.
In your second equation, both of those numbers are constrained to whatever
type i is, and fact demands an Integer and (**) demands a Float or Double
or some other Floating type, which cannot be reconciled.
You should be able to fix this with (untested) x**fromIntegral(i), which
converts i from an Integer to Num and all Floating are Nums.
On Wed, Nov 30, 2022, 19:41 Jonathan Drews
Hi Folks:
I am using Glasgow Haskell Compilation System, version 9.2.4 on OpenBSD 7.2. I am stumped by the following problem. If I run the following program in ghci, then it works fine. Look:
fact :: Integer -> Integer fact n = product [1..n]
term :: Double -> Double term x = x**4/fromIntegral(fact(4))
$ ghci GHCi, version 9.2.4: https://www.haskell.org/ghc/ :? for help ghci> :l seriesTermTest.hs [1 of 1] Compiling Main ( seriesTermTest.hs, interpreted ) Ok, one module loaded. ghci> term 1.0 4.1666666666666664e-2
However if I use fromIntegral inside a list comprehension like so:
fact :: Integer -> Integer fact n = product [1..n]
expon :: Double -> Double expon x = sum [x**i/fromIntegral(fact(i)) | i <- [0..50]]
then I get the following error message:
$ ghci GHCi, version 9.2.4: https://www.haskell.org/ghc/ :? for help ghci> :l ePowerSeries.hs [1 of 1] Compiling Main ( ePowerSeries.hs, interpreted )
ePowerSeries.hs:6:40: error: * Couldn't match expected type `Integer' with actual type `Double' * In the first argument of `fact', namely `(i)' In the first argument of `fromIntegral', namely `(fact (i))' In the second argument of `(/)', namely `fromIntegral (fact (i))' | 6 | expon x = sum [x**i/fromIntegral(fact(i)) | i <- [0..50]] | ^ Failed, no modules loaded.
What am I doing wrong?
-- Kind regards, Jonathan
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