[Haskell-beginners] annoying precedence of unary negate

[Magnus Therning]

On Mon, Apr 28, 2014 at 11:30 AM, John M. Dlugosz
<ngnr63q02 at sneakemail.com> 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: magnus at therning.org   jabber: magnus at therning.org
twitter: magthe               http://therning.org/magnus