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Today's Topics:
1. is this right? (Dennis Raddle)
2. Re: is this right? (Sylvain Henry)
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Message: 1
Date: Thu, 22 Sep 2016 02:57:17 -0700
From: Dennis Raddle <dennis.rad...@gmail.com>
To: Haskell Beginners <beginners@haskell.org>
Subject: [Haskell-beginners] is this right?
Message-ID:
<CAKxLvoo4AuA=3Y9FVBW6qMmJm3N8KshE7krX=n6wwpq+++p...@mail.gmail.com>
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I want to evaluate a function on a series of inputs:
f :: a -> Maybe b
then collect the results [b] if they are all Just, or terminate the
computation immediately upon hitting Nothing.
This is exactly what mapM does in the Maybe monad, correct?
In particular I want to make sure that it will not try to evaluate anything
past the first 'Nothing' result as the efficiency of my design is based on
that.
D
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Message: 2
Date: Thu, 22 Sep 2016 12:33:11 +0200
From: Sylvain Henry <sylv...@haskus.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] is this right?
Message-ID: <44e48242-044f-6d01-4488-8e06cdc54...@haskus.fr>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Hi,
Yes it's correct. You can check this with ghci:
f :: Integer -> Maybe Integer
f 5 = Nothing
f x = Just x
> let xs = [1..] :: [Integer]
> mapM f xs
Nothing (it doesn't loop forever)
> :sprint xs
xs = 1 : 2 : 3 : 4 : 5 : _ (the tail after 5 is not evaluated)
Cheers
Sylvain
On 22/09/2016 11:57, Dennis Raddle wrote:
> I want to evaluate a function on a series of inputs:
>
> f :: a -> Maybe b
>
> then collect the results [b] if they are all Just, or terminate the
> computation immediately upon hitting Nothing.
>
> This is exactly what mapM does in the Maybe monad, correct?
>
> In particular I want to make sure that it will not try to evaluate
> anything past the first 'Nothing' result as the efficiency of my
> design is based on that.
>
> D
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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