[Haskell-cafe] I need help getting started

mitchell at kaplan2.com mitchell at kaplan2.com
Sun Apr 25 11:47:32 EDT 2010



Thanks, I didn't know it was there.  I'll subscribe to it.





From: John Bender [mailto:john.m.bender at gmail.com] 
Sent: Sunday, April 25, 2010 3:01 AM
To: Luke Palmer
Cc: mitchell at kaplan2.com; haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] I need help getting started




You might also be interested in the beginners mailing list. I've been
enjoying for about a month now! 




On Sat, Apr 24, 2010 at 9:57 PM, Luke Palmer <lrpalmer at gmail.com> wrote:

On Sat, Apr 24, 2010 at 10:34 PM,  <mitchell at kaplan2.com> wrote:
> Hi,
> I'm just starting to learn, or trying to learn Haskell.  I want to write a
> function to tell me if a number's prime.  This is what I've got:
> f x n y = if n>=y
>           then True
>           else
>           if gcd x n == 1
>           then f x (n+1) y
>           else False
> primeQ x = f x 2 y
>   where y = floor(sqrt(x))

Pretty good so far.  The only trouble is that the type of x is
inconsistent. In f it is an integer, but in primeQ it is a floating
point (because you are taking its square root).  Getting past this
just involves understanding Haskell's type system peculiarities.
Change that last line to:

   where y = floor (sqrt (fromIntegral x))

And you should be fine.  (Untested)

In the future, post the error that you are getting addition to the
code that is causing it.  That helps us find it faster.


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