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Today's Topics:
1. Adding 1 to Just 9 (Olumide)
2. Re: Adding 1 to Just 9 (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Mon, 14 May 2018 13:15:31 +0100
From: Olumide <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Adding 1 to Just 9
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Dear List,
Chapter 14 of LYH appears to suggest that a Just value can be added to
an Int. Quote from
http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions
"For instance, say we have Just 9 and the function \x -> Just (x+1). If
we map this function over Just 9, we're left with Just (Just 10)."
I've tried the following in ghci but got the error:
<interactive>:12:1: error:
• Non type-variable argument in the constraint: Num (Maybe a)
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. (Num (Maybe a), Num a) => Maybe a
Am I reading the quote wrong? Is Just (Just 10) a hypothetical?
Regards,
- Olumide
------------------------------
Message: 2
Date: Mon, 14 May 2018 08:18:31 -0400
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Adding 1 to Just 9
Message-ID:
<can+tr41yylwfc0157tm_fsg05z6mwio8g4imeyp3qxcf-oy...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
let foo = \x -> Just (x + 1)
fmap foo (Just 9)
Just (Just 10)
On Mon, May 14, 2018 at 8:15 AM, Olumide <[email protected]> wrote:
> Dear List,
>
> Chapter 14 of LYH appears to suggest that a Just value can be added to an
> Int. Quote from http://learnyouahaskell.com/fo
> r-a-few-monads-more#useful-monadic-functions
>
> "For instance, say we have Just 9 and the function \x -> Just (x+1). If we
> map this function over Just 9, we're left with Just (Just 10)."
>
> I've tried the following in ghci but got the error:
>
> <interactive>:12:1: error:
> • Non type-variable argument in the constraint: Num (Maybe a)
> (Use FlexibleContexts to permit this)
> • When checking the inferred type
> it :: forall a. (Num (Maybe a), Num a) => Maybe a
>
>
> Am I reading the quote wrong? Is Just (Just 10) a hypothetical?
>
> Regards,
>
> - Olumide
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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