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Today's Topics:
1. Conditionals where more than one case is true (PATRICK BROWNE)
2. Re: Conditionals where more than one case is true
(Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Mon, 17 Apr 2017 11:26:07 +0100
From: PATRICK BROWNE <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Conditionals where more than one case is
true
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<CAGFLrKcBV0yS3PimtEGzMYPG=Mdu=7z272vgc9mjnd4edhr...@mail.gmail.com>
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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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Message: 2
Date: Mon, 17 Apr 2017 12:54:12 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Conditionals where more than one case
is true
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
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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