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Today's Topics:
1. Re: annoying precedence of unary negate (John M. Dlugosz)
2. Re: annoying precedence of unary negate (John M. Dlugosz)
3. Re: annoying precedence of unary negate (Magnus Therning)
----------------------------------------------------------------------
Message: 1
Date: Mon, 28 Apr 2014 04:13:29 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] annoying precedence of unary negate
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 4/28/2014 2:08 AM, Kim-Ee Yeoh wrote:
>
> On Mon, Apr 28, 2014 at 10:59 AM, John M. Dlugosz <[email protected]
> <mailto:[email protected]>> wrote:
>
> Also, is the comma in a list different from the operator comma which is
> noted as
> right-associative precedence 5? I had thought that the item separator in
> the list was
> special syntax that had very low precedence.
>
>
> The _colon_ is a non-rebindable special-syntax operator with infixr 5.
>
> Otoh, the _comma_ in a list is _not_ an operator but merely an item separator.
>
> Comma-separated list notation e.g. [1,2,3] is special syntax that desugars to
> e.g. 1:2:3:[].
>
> Other than that, the colon and comma don't have anything else in common.
>
> -- Kim-Ee
>
>
I misread the chart: the ?:,++? meant to use the comma to separate two
operators, not be
one of three operators listed.
------------------------------
Message: 2
Date: Mon, 28 Apr 2014 04:30:49 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] annoying precedence of unary negate
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 4/27/2014 11:24 PM, Magnus Therning wrote:
> On Sun, Apr 27, 2014 at 10:59:27PM -0500, John M. Dlugosz wrote:
>>
>
> It most likely sees it as the binary minus rather than the unary minus.
>
That's what I thought.
Couldn't match expected type `Picture'
with actual type `Float -> Picture -> Picture'
In the expression: translate - 50 0 $ color green (Circle 50)
In the first argument of `Pictures', namely
`[Circle 100, translate - 50 0 $ color green (Circle 50),
scale 200 200 $ color red $ (Pictures $ take 20 chain1), testcirc]'
In the expression:
Pictures
[Circle 100, translate - 50 0 $ color green (Circle 50),
scale 200 200 $ color red $ (Pictures $ take 20 chain1), testcirc]
But what is the error message telling me?
Given that infix is done after adjacency application, it should parse as:
((translate - (50 0) ) $ (color (green (Circle 50))))
Left of the $, that is the parse tree
subtract+
|
+ translate
|
+ apply+
|
+ 50
|
+ 0
I think it would complain that 50 isn't a function, or the first argument of
subtract is
not a Num but a function
translate :: Float -> Float -> Picture -> Picture,
or that the argument of translate isn't a Float but something it can't make
sense of.
Why is it looking for a Picture? Where is it getting Float->Picture->Picture
(seems to be
a curried translate? But the next token is not something it would like so how
can it find
a first argument?)
Understanding the compiler's errors is a skill I want to learn, as well as
shake out my
understanding of what's really going on.
Thanks,
?John
------------------------------
Message: 3
Date: Mon, 28 Apr 2014 12:43:10 +0200
From: Magnus Therning <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] annoying precedence of unary negate
Message-ID:
<CAAExw5t4q-0P79gswTm6c4MyqZPr7-nsNbqN9dAmqis=0u2...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Mon, Apr 28, 2014 at 11:30 AM, John M. Dlugosz
<[email protected]> wrote:
> On 4/27/2014 11:24 PM, Magnus Therning wrote:
>> It most likely sees it as the binary minus rather than the unary minus.
>
> That's what I thought.
>
> Couldn't match expected type `Picture'
> with actual type `Float -> Picture -> Picture'
> In the expression: translate - 50 0 $ color green (Circle 50)
> In the first argument of `Pictures', namely
> `[Circle 100, translate - 50 0 $ color green (Circle 50),
> scale 200 200 $ color red $ (Pictures $ take 20 chain1), testcirc]'
> In the expression:
> Pictures
> [Circle 100, translate - 50 0 $ color green (Circle 50),
> scale 200 200 $ color red $ (Pictures $ take 20 chain1), testcirc]
>
> But what is the error message telling me?
> Given that infix is done after adjacency application, it should parse as:
> ((translate - (50 0) ) $ (color (green (Circle 50))))
>
> Left of the $, that is the parse tree
>
> subtract+
> |
> + translate
> |
> + apply+
> |
> + 50
> |
> + 0
>
> I think it would complain that 50 isn't a function, or the first argument of
> subtract is not a Num but a function
> translate :: Float -> Float -> Picture -> Picture,
> or that the argument of translate isn't a Float but something it can't make
> sense of.
> Why is it looking for a Picture? Where is it getting
> Float->Picture->Picture (seems to be a curried translate? But the next
> token is not something it would like so how can it find a first argument?)
>
> Understanding the compiler's errors is a skill I want to learn, as well as
> shake out my understanding of what's really going on.
It's not easy to answer that without knowing the types of the involved
values/functions. You can always play around a bit in ghci with the
':type' command to see if you can work it out for yourself :)
It can be confusing though, functions are values too, and precedence
rules come into play too.
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: [email protected] jabber: [email protected]
twitter: magthe http://therning.org/magnus
------------------------------
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