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Today's Topics:
1. Re: Question regarding error messages (Francesco Ariis)
2. Re: Question regarding error messages (David CAI)
3. Re: Question regarding error messages (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Sun, 18 Jun 2017 18:49:09 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Question regarding error messages
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On Sun, Jun 18, 2017 at 05:02:13PM +0800, Choi Waikit wrote:
> Hi guys,
>
> I have difficulty understanding how GHC determines what kind of error
> message to return. Could you help explain a bit ? Thanks !
>
> The code can be accessed here <http://lpaste.net/356325>
>
> My question is
> Why does calling `settleDown 9` return different error message from
> `settleDown "sting"` ?
Hello David,
every time you write a number in a haskell source, it gets automagically
wrapped in `fromInteger` (actually, `fromInteger` if there isn't a dot,
`fromRational` otherwise). This is made to provide some kind of overloading.
`fromInteger` has signature:
λ> :t fromInteger
fromInteger :: Num a => Integer -> a
hence the different error with 9 and "string"!
Is it clear now?
------------------------------
Message: 2
Date: Mon, 19 Jun 2017 10:56:59 +0800
From: David CAI <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Question regarding error messages
Message-ID:
<CAJ7MPFoUnmXeecEWLzEdAbdCE6Mj_85kzEdgCAyr8=i2-hx...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi Francesco,
Thanks for your reply! Just to make sure I understood, can I paraphrase
your words as the following ?
In the case of `settleDown 9`
`settleDown 9` is first translated to `settleDown $ fromInteger 9`. Since
fromInteger lies in Num type class and settleDown's signature is:
settleDown :: Mood -> Mood, the compiler tries to look for
an instance for Num typeclass so that it can resolve which `fromInteger`
method to call. Therefore it doesn't show a type mismatch error but hint
for implementing a typeclass.
In the case of `settleDown "hello"`
There's no preprocessing for "hello" so its type remains [Char]. Since
settleDown has type Mood -> Mood, Mood and [Char] are not compatible,
therefore the compiler shows a type mismatch error.
Thanks,
David
On 19 June 2017 at 00:49, Francesco Ariis <[email protected]> wrote:
> On Sun, Jun 18, 2017 at 05:02:13PM +0800, Choi Waikit wrote:
> > Hi guys,
> >
> > I have difficulty understanding how GHC determines what kind of error
> > message to return. Could you help explain a bit ? Thanks !
> >
> > The code can be accessed here <http://lpaste.net/356325>
> >
> > My question is
> > Why does calling `settleDown 9` return different error message from
> > `settleDown "sting"` ?
>
> Hello David,
> every time you write a number in a haskell source, it gets
> automagically
> wrapped in `fromInteger` (actually, `fromInteger` if there isn't a dot,
> `fromRational` otherwise). This is made to provide some kind of
> overloading.
>
> `fromInteger` has signature:
>
> λ> :t fromInteger
> fromInteger :: Num a => Integer -> a
>
> hence the different error with 9 and "string"!
>
> Is it clear now?
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 3
Date: Mon, 19 Jun 2017 07:54:07 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Question regarding error messages
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Mon, Jun 19, 2017 at 10:56:59AM +0800, David CAI wrote:
> Hi Francesco,
>
> Thanks for your reply! Just to make sure I understood, can I paraphrase
> your words as the following ?
>
> In the case of `settleDown 9`
> `settleDown 9` is first translated to `settleDown $ fromInteger 9`. Since
> fromInteger lies in Num type class and settleDown's signature is:
> settleDown :: Mood -> Mood, the compiler tries to look for
> an instance for Num typeclass so that it can resolve which `fromInteger`
> method to call. Therefore it doesn't show a type mismatch error but hint
> for implementing a typeclass.
>
> In the case of `settleDown "hello"`
> There's no preprocessing for "hello" so its type remains [Char]. Since
> settleDown has type Mood -> Mood, Mood and [Char] are not compatible,
> therefore the compiler shows a type mismatch error.
Excellent explanation, that's exactly it!
------------------------------
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