[Haskell-cafe] I need help getting started

mitchell at kaplan2.com mitchell at kaplan2.com
Sun Apr 25 11:46:59 EDT 2010

Hi Luke,

Your fix worked.  Thanks very much.  I guess I thought that x was local to
primeQ and when I called f, with y, it was just passing a value.  As I was
this response to you, I realized that I'm also passing x itself, oops.  So I
tried setting z=floor x, and passing z instead of x, and that also worked!
A little light is beginning to glimmer.

This will take a little getting used to.  Thanks very much.


-----Original Message-----
From: Luke Palmer [mailto:lrpalmer at gmail.com] 
Sent: Sunday, April 25, 2010 12:58 AM
To: mitchell at kaplan2.com
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] I need help getting started

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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