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Today's Topics:
1. GHCi error msgs (Julius Gedvilas)
2. Re: GHCi error msgs (Karolis Velicka)
3. Re: GHCi error msgs (Julius Gedvilas)
4. Question on syntax (Rohit Sharma)
5. Re: Question on syntax (Bob Ippolito)
6. Re: Question on syntax (Rustom Mody)
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Message: 1
Date: Fri, 24 Oct 2014 16:30:36 +0300
From: Julius Gedvilas <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] GHCi error msgs
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How difficult would it be to change format of GHCi compile-time
error-messages of type "couldn't match expected type"?
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Message: 2
Date: Fri, 24 Oct 2014 16:56:35 +0100
From: Karolis Velicka <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] GHCi error msgs
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What do you mean by that? Can you give an example?
Best wishes,
Karl (Karolis) Velicka
On 24 October 2014 14:30, Julius Gedvilas <[email protected]> wrote:
> How difficult would it be to change format of GHCi compile-time
> error-messages of type "couldn't match expected type"?
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 3
Date: Fri, 24 Oct 2014 19:10:59 +0300
From: Julius Gedvilas <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] GHCi error msgs
Message-ID:
<caowifxllx+ez45gxuxyrwdhaajlzsfgr0gq0hx4wbb+rqku...@mail.gmail.com>
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You load a *.hs file into ghci (cabal repl) and, if types does not add up,
you get an error message in form of "*.hs:line:col: Couldn't match expected
type <Type> with actual type <Type> ...".
I may like to reformat that message with Parsec.
On Fri, Oct 24, 2014 at 6:56 PM, Karolis Velicka <[email protected]>
wrote:
> What do you mean by that? Can you give an example?
>
> Best wishes,
> Karl (Karolis) Velicka
>
>
> On 24 October 2014 14:30, Julius Gedvilas <[email protected]> wrote:
> > How difficult would it be to change format of GHCi compile-time
> > error-messages of type "couldn't match expected type"?
> >
> > _______________________________________________
> > Beginners mailing list
> > [email protected]
> > http://www.haskell.org/mailman/listinfo/beginners
> >
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 4
Date: Sat, 25 Oct 2014 11:31:01 +0800
From: Rohit Sharma <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Question on syntax
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All,
I started learning haskell very recently and have a question on the pattern
matching style for lists.
In the below snippet i.e. "(x:xs)" why do we went with round braces and not
square? I know we are using cons that tells this is not a tuple but would
it not make more sense to write something like [x:xs] instead of (x:xs), i
thought round braces was used for pair/tuples?
safeHead [] = Nothing
safeHead (x:xs) = Just x
Thanks,
Rohit
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Message: 5
Date: Fri, 24 Oct 2014 20:41:11 -0700
From: Bob Ippolito <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Question on syntax
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Round braces are used for grouping, it's necessary to avoid ambiguity since
a function can be defined with more than one argument. When using case you
don't have this potential ambiguity, so don't need the parentheses. Tuples
are more of a special case in the grammar that isn't closely related to
this.
safeHead p = case p of
[] -> Nothing
x : xs -> Just x
On Friday, October 24, 2014, Rohit Sharma <[email protected]> wrote:
> All,
>
> I started learning haskell very recently and have a question on the
> pattern matching style for lists.
>
> In the below snippet i.e. "(x:xs)" why do we went with round braces and
> not square? I know we are using cons that tells this is not a tuple but
> would it not make more sense to write something like [x:xs] instead of
> (x:xs), i thought round braces was used for pair/tuples?
>
> safeHead [] = Nothing
> safeHead (x:xs) = Just x
>
> Thanks,
> Rohit
>
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Message: 6
Date: Sat, 25 Oct 2014 09:35:57 +0530
From: Rustom Mody <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Question on syntax
Message-ID:
<caj+teodnxkvvavgtx7_6vusnoa51s_aqnu5ygzwhmqhwdbk...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sat, Oct 25, 2014 at 9:01 AM, Rohit Sharma <[email protected]> wrote:
> All,
>
> I started learning haskell very recently and have a question on the
> pattern matching style for lists.
>
> In the below snippet i.e. "(x:xs)" why do we went with round braces and
> not square? I know we are using cons that tells this is not a tuple but
> would it not make more sense to write something like [x:xs] instead of
> (x:xs), i thought round braces was used for pair/tuples?
>
> safeHead [] = Nothing
> safeHead (x:xs) = Just x
>
>
Yes this can be confusing.
Lets break it into two separate questions:
1. Why [] is not used around x:xs
2. Why () is used
To address 1 start ghci and try out these expressions
1: [2,3]
[1:[2,3]]
To address 2 try out
length [1,2] ++ [3,4]
and then
length ([1,2] ++ [3,4])
Another related example
sin pi/2
and
sin (pi/2)
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