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Today's Topics:
1. Question on fromIntegral (Jonathan Drews)
2. Re: Question on fromIntegral (David McBride)
3. Re: Question on fromIntegral (Jonathan Drews)
----------------------------------------------------------------------
Message: 1
Date: Wed, 30 Nov 2022 17:41:03 -0700
From: Jonathan Drews <jondr...@fastmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Question on fromIntegral
Message-ID: <y4f4h6025zvta...@leo.my.domain>
Content-Type: text/plain; charset=us-ascii
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
------------------------------
Message: 2
Date: Wed, 30 Nov 2022 23:05:21 -0500
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Question on fromIntegral
Message-ID:
<can+tr41zqefn0u9mabcyxbcdpoypqq+okdqnlr3et2xhwho...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <jondr...@fastmail.com> wrote:
> 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
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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------------------------------
Message: 3
Date: Wed, 30 Nov 2022 22:17:49 -0700
From: Jonathan Drews <jondr...@fastmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Question on fromIntegral
Message-ID: <Y4g4/ewffhxio...@leo.my.domain>
Content-Type: text/plain; charset=us-ascii
On Wed, Nov 30, 2022 at 11:05:21PM -0500, David McBride wrote:
> 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.
Thank you David:
I switched from x**1 to x^i and it worked
fact :: Integer -> Integer
fact n = product [1..n]
expon :: Double -> Double
expon x = sum [x^i/fromIntegral(fact(i)) | i <- [0..50]]
$ 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 )
Ok, one module loaded.
ghci> exp
exp expon exponent
ghci> expon 2
7.389056098930649
ghci> exp 2
7.38905609893065
I believe x^i has a similar effect to x**fromIntegral(i)
--
Kind regards,
Jonathan
------------------------------
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