Oh, Sorry. Have been away from email today. (Focused 100% in VS-code on a
cool feature for Go)
My statement was meant to convey an insight into a subtle aspect of Go's
constants and high-precision expression evaluation--that the "type" of
untyped constants/values, in the conversion rules sense, is derived in the
instant of the conversion and is not an inherent trait of the value as is
quite generally the case in other languages and situations. Also I think,
the spec document is a little deceptive here, or at least, it needs to be
read creatively to understand what all the possibilities are.
In this case, the original question by Djadala was about a const "6.6" not
being able to be coerced to integer form in either of two ways of
trying ("x = int(y)" and "x int = y"). Both you and Jan explained why this
is so and where/how it is defined in the specification. That was
authoritative.
However, the original question was worded in such a way that I could not
discern if he was asking "please share the official reason why this seems
impossible" or if instead he/she meant, "can I do this? I've tried two
ways, and it seems impossible." I'm very supportive of the "how can I make
it work?" sentiment and I doubted that the OP would figure it out since the
two responses seemed so absolute, so I weighed in.
Here's the subtle point. *That 6.6 constant "y" is NOT a floating-point
value.* it's value just happens to "be as if floating" at the moment.
However, "y*10" evaluates to 66, and that is a "just plain 66" not "10
times the floating point 6.6, therefore an inherent floating point value of
66.0" The upshot is that Djadala cannot get "y" into an int, but can easily
get "10*y" into one, and having done so, can make that integer 66 into the
integer 6 via truncating integer division as I showed in the linked Go
Playground example: https://play.golang.org/p/7gcb9Yv7c9e
Since the "6.6" has one decimal place making y *be as if floating point,*
multiplication by an integer multiple of 10 is enough to clear the decimals
and let it *be as if integer* in arbitrary expressions. If it was "6.67" (2
places) or "6.678" (three places), then the least power of ten to clear the
"as if floating" problem Djadala faced would be "10**k, k >= number of
decimals" as was indicated in my comment in the linked Playground example.
Sadly, this is not a general-purpose, remedy because the magic expression
evaluation logic insists that values fit into the target size; were it not
this way, we might imagine multiplying by 10**k, k>=19, capturing the
integerness of the expression in that instant, and then dividing by the
same factor. Alas, we'll be stopped right there because the expression,
while *as if an integer,* is simultaneously, *as if too big to fit into
one. *Since 64-bit ints are big and the OP was wrestling with only one
decimal place in a small sized number, it worked with fifteen or more
digits of precision to spare. However, if "y" were (1e16 + 0.6) then there
would not be much wiggle room to clear the decimals this way.
If these kinds of topics are interesting to you, there are discussions
about related matters and possible changes for Go 2 in various proposal
threads.
On Mon, May 20, 2019 at 11:50 PM <fge...@gmail.com> wrote:
> Could you please clarify which statement your example is meant to
> confirm or refute?
>
>
>
> On 5/20/19, Michael Jones <michael.jo...@gmail.com> wrote:
> > ahem...
> > https://play.golang.org/p/7gcb9Yv7c9e
> >
> > On Mon, May 20, 2019 at 2:45 AM <fge...@gmail.com> wrote:
> >
> >> https://golang.org/ref/spec#Constants
> >>
> >> "A constant may be given a type explicitly by a constant declaration
> >> or conversion, or implicitly when used in a variable declaration or an
> >> assignment or as an operand in an expression. It is an error if the
> >> constant value cannot be represented as a value of the respective
> >> type."
> >>
> >>
> >>
> >> On 5/20/19, djad...@gmail.com <djad...@gmail.com> wrote:
> >> > Hi,
> >> > does this should work ?
> >> >
> >> > package main
> >> >
> >> > import (
> >> > "fmt"
> >> > )
> >> >
> >> > func main() {
> >> > const (
> >> > c = 2
> >> > z = 3.3
> >> > y = c * z
> >> > x = int(y) // or x int = y
> >> > )
> >> >
> >> > fmt.Println(x)
> >> > }
> >> >
> >> >
> >> > it does not compile with error:
> >> > constant 6.6 truncated to integer
> >> >
> >> > https://play.golang.org/p/f-Q6XSvvcrM
> >> >
> >> > --
> >> > You received this message because you are subscribed to the Google
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> >> > "golang-nuts" group.
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> >> > an
> >> > email to golang-nuts+unsubscr...@googlegroups.com.
> >> > To view this discussion on the web visit
> >> >
> >>
> https://groups.google.com/d/msgid/golang-nuts/736e8b03-d38f-4ec4-adc8-fcb77e0cda7b%40googlegroups.com
> >> .
> >> > For more options, visit https://groups.google.com/d/optout.
> >> >
> >>
> >> --
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> >
> >
> > --
> >
> > *Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
> >
>
--
*Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
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