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Today's Topics:
1. Value from Monad (Shishir Srivastava)
2. Re: Value from Monad
(Sumit Sahrawat, Maths & Computing, IIT (BHU))
3. Re: Value from Monad (Brandon Allbery)
4. Re: Value from Monad (Shishir Srivastava)
5. Re: Value from Monad (Mike Meyer)
6. Re: Value from Monad (Brandon Allbery)
7. Re: Value from Monad (Marcin Mrotek)
----------------------------------------------------------------------
Message: 1
Date: Wed, 22 Apr 2015 16:24:23 +0100
From: Shishir Srivastava <[email protected]>
To: beginners <[email protected]>
Subject: [Haskell-beginners] Value from Monad
Message-ID:
<cale5rtu2watosu4vstkz+5pnmyfkujbwdmcrsbbg-sedmzn...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
Going through the '<-' description in 'learnouahaskell' it says -
*" If we have a Maybe String and we bind it with <- to a variable, that
variable will be a String"*
so if you write
do
str <- Just "3"
str should be of type 'String'. My question is how do I return "34"
by concatenating it to "4" because the following line of code fails -
---
do
str <- Just "3"
return str++"4"::Maybe String
---
or for that matter how can I return/print just the value of str from this
do block ?
Thanks,
Shishir
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Message: 2
Date: Wed, 22 Apr 2015 20:55:45 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<CAJbEW8MAGON0543rK8MAYKTw9PW0Omb=x8s2fashwwoaszd...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
return (str++"4") should work
On 22 April 2015 at 20:54, Shishir Srivastava <[email protected]>
wrote:
> Hi,
>
> Going through the '<-' description in 'learnouahaskell' it says -
>
> *" If we have a Maybe String and we bind it with <- to a variable, that
> variable will be a String"*
>
> so if you write
>
> do
> str <- Just "3"
>
> str should be of type 'String'. My question is how do I return "34"
> by concatenating it to "4" because the following line of code fails -
>
> ---
> do
> str <- Just "3"
> return str++"4"::Maybe String
> ---
>
> or for that matter how can I return/print just the value of str from this
> do block ?
>
> Thanks,
> Shishir
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
--
Regards
Sumit Sahrawat
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Message: 3
Date: Wed, 22 Apr 2015 11:28:16 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<CAKFCL4XoCif5_hW_+C+Q3gtzaf=iS=pnzljhm+olvntretw...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Wed, Apr 22, 2015 at 11:24 AM, Shishir Srivastava <
[email protected]> wrote:
> return str++"4"::Maybe String
>
This is parsed as "(return str) ++ "4". Function application is always
higher precedence than operators.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 4
Date: Wed, 22 Apr 2015 16:38:34 +0100
From: Shishir Srivastava <[email protected]>
To: beginners <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<cale5rts7l5ugcspckona4-k7p-p0n5eryd4zrfs14f0rfic...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thanks ! that does work now.
My next question was how do i only get "34" or "3" i.e. the Maybe value
without wrapped in to a 'Maybe' ?
Shishir
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Message: 5
Date: Wed, 22 Apr 2015 10:41:15 -0500
From: Mike Meyer <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<CAD=7u2bhoj+86gdb1mdwnryzqgx4kexdft3xc3zpneye-r6...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You should use something like the maybe function to extract it after you've
returned the Just value.
On Wed, Apr 22, 2015 at 10:38 AM, Shishir Srivastava <
[email protected]> wrote:
> Thanks ! that does work now.
>
> My next question was how do i only get "34" or "3" i.e. the Maybe value
> without wrapped in to a 'Maybe' ?
>
> Shishir
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Message: 6
Date: Wed, 22 Apr 2015 11:43:38 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<cakfcl4veevg9n0iqxdezjhvyxt8ryqvuyp3y6hckmrjd8nq...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Wed, Apr 22, 2015 at 11:38 AM, Shishir Srivastava <
[email protected]> wrote:
> My next question was how do i only get "34" or "3" i.e. the Maybe value
> without wrapped in to a 'Maybe' ?
"do" syntax lets you temporarily "unwrap" the value, but it must be
rewrapped later. And if the specific monad in question is not IO, you
cannot do I/O (e.g. "print" the value as in your initial message). This
reflects the mapping of "do" to uses of (>>=)
Prelude> :t (>>=)
(>>=) :: Monad m => m a -> (a -> m b) -> m b
In this example, `m` is Maybe.
As it turns out, Maybe is one of the monads that lets you use pattern
matching to extract values. Not all monads do; you cannot do this with IO,
for example. The `maybe` function is often used instead of explicit pattern
matching.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 7
Date: Wed, 22 Apr 2015 22:23:36 +0200
From: Marcin Mrotek <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Value from Monad
Message-ID:
<cajcfpzkbk6pcuyalaepu5_7ksufwtw5fjanffi9o7updnyf...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Hello,
Well, you CAN use fromJust, but bear in mind that you should only use
it if you are absolutely, positively, 100% sure the Maybe value you
are calling fromJust on is of the form (Just something), because your
program will error out if you call fromJust on a Nothing. Even if
you're sure it's not a Nothing, it might be better to use
maybe (error "Impossible, because (...)") id
instead, so that you at the very least get a meaningful error message
in case "impossible" happens ;) Other than that, it's recomennded to
either use the maybe function, or pattern matching:
case foo of
Just x -> ...
Nothing -> ...
instead.
Best regards,
Marcin Mrotek
------------------------------
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