Conditionals where more than one case is true

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Conditionals where more than one case is true

Patrick Browne
I am having difficulty in evaluating conditionals where more than one case is true.
I require a program to express the following conditions:
 1) x is faster than y if x is a buffolo and y is a pig.
 2) x is faster than y if x is a pig and y is a slug.
Both of these conditions are true (expressed in fact predicate).
I include my attempt using guards (I have also tried case/if-then-else)

I can apreciate that the program works when the first guard condition is met (faster Bob Joe).
I am not sure why it fails on the second case (faster Bob Joe)

Can I program the requirement using conditionals?
Can I pattern match is some way?
Is there another way to encode the requirement.
Regards,
Pat


data E = Bob | Joe | Steve | Buffalo | Pig | Slug  deriving Show

fact Buffalo Bob  = True 
fact Pig Joe  = True 
fact Slug Steve  = True 


faster x y | fact Buffalo x && fact Pig y =  True 
           | fact Pig x && fact Slug y =  True 
           | otherwise = False
-- faster Bob Joe OK
-- faster Steve  Bob ***Exception: Faster.hs:(5,1)-(7,23): Non-exhaustive patterns in function fact

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Re: Conditionals where more than one case is true

Francesco Ariis
On Mon, Apr 17, 2017 at 11:26:07AM +0100, PATRICK BROWNE wrote:
> I can apreciate that the program works when the first guard condition is
> met (faster Bob Joe).
> I am not sure why it fails on the second case (faster Bob Joe)

Your `fact` function is the one that is partial

    fact Buffalo Bob = True
    fact Pig Joe     = True
    fact Slug Steve  = True
    -- what if all those patterns fail?

You should add a line to handle "every other case", like

    fact Buffalo Bob = True
    fact Pig Joe     = True
    fact Slug Steve  = True
    fact _    _      = False

Does that help?
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Re: Conditionals where more than one case is true

Patrick Browne
  Francesco,
 Yes, what I was looking for.
Thanks,
Pat

On 17 April 2017 at 11:54, Francesco Ariis <[hidden email]> wrote:
On Mon, Apr 17, 2017 at 11:26:07AM +0100, PATRICK BROWNE wrote:
> I can apreciate that the program works when the first guard condition is
> met (faster Bob Joe).
> I am not sure why it fails on the second case (faster Bob Joe)

Your `fact` function is the one that is partial

    fact Buffalo Bob = True
    fact Pig Joe     = True
    fact Slug Steve  = True
    -- what if all those patterns fail?

You should add a line to handle "every other case", like

    fact Buffalo Bob = True
    fact Pig Joe     = True
    fact Slug Steve  = True
    fact _    _      = False

Does that help?
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Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie

Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman


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