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.
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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