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cmd/compile: possible error message improvement #4565

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rsc opened this Issue Dec 18, 2012 · 6 comments

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rsc commented Dec 18, 2012

http://play.golang.org/p/87zosrj611

package main

var x = 10
var y = 0.9 * x

prog.go:4: constant 0.9 truncated to integer

If you don't know that x is an int or that * requires both sides to have the same type,
this is a bit of a leap.

constant 0.9 truncated during conversion to type int (to match x)

might be a little better.
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rsc commented Mar 12, 2013

Comment 1:

[The time for maybe has passed.]

Labels changed: removed go1.1maybe.

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rsc commented Jul 30, 2013

Comment 2:

Labels changed: added go1.2maybe.

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rsc commented Sep 9, 2013

Comment 3:

Labels changed: added go1.3maybe, removed go1.2maybe.

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rsc commented Dec 4, 2013

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

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rsc commented Dec 4, 2013

Comment 5:

Labels changed: added repo-main.

@rsc rsc added accepted labels Dec 4, 2013

@rsc rsc self-assigned this Dec 4, 2013

@rsc rsc added this to the Unplanned milestone Apr 10, 2015

@rsc rsc changed the title cmd/gc: possible error message improvement cmd/compile: possible error message improvement Jun 8, 2015

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nathany commented Mar 4, 2016

I agree that it's a bit of a leap.

constant 0.9 truncated to integer

Compare that to the error when only variables are in the expression. It includes the expression and the types of the variables in question: https://play.golang.org/p/nfZ_xMultw

invalid operation: w * x (mismatched types float64 and int)

I like the idea of including (x) in the error message.

constant truncated: 0.9 * x ...

On the other hand, it's not always clear which variables are the cause when only variables are involved: https://play.golang.org/p/9_EchugdkA

invalid operation: v * w * x (mismatched types float64 and int)

So maybe there's a better way to do that error message as well. What comes to mind is the conversion syntax to indicate the types:

constant 0.9 truncated to integer: 0.9 * int(x)
mismatched types: float64(v) * float64(w) * int(x)

Here's a slightly more complex constants example where it might be nice if the error indicated that w is involved as well as that x is an integer (the reason for the truncation)

const w = 0.9
var x = 10
var z = w * w * x

constant 0.81 truncated to integer
constant 0.81 truncated to integer: w * w * int(x)
constant 0.81 truncated to integer: 0.9 * 0.9 * int(x)

https://play.golang.org/p/wklLBzn6XY

This is my random issue today via CodeTriage.

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