4 ms·

I think this is wrong or at least misleading. According to the Ecma standard: Boolean conversion maps any object o into a boolean. Boolean conversion is defi

by berns 12y ago

I think this is wrong or at least misleading. According to the Ecma standard: Boolean conversion maps any object o into a boolean. Boolean conversion is defined by the function application (bool v){ assert(v != null); return identical(v, true); }(o)

dragonwriter 12y ago
Reading your quote from the ECMA standard, "no implicit conversions are supported" may be wrong but at the same time not generally misleading. That is, that function errors on null values and returns false for anything that isn't identical to true, so while it does provide a conversion to boolean from any non-null value, anything that isn't exactly a boolean "true" (unless I misunderstand what "identical" does) is going to return false on conversion. This is consistent with other writings on booleans in Dart [1]. So it appears that Dart has implicit boolean conversions, but it converts everything that is not the single "true" value to false, so using an expression that never can evaluate exactly to boolean true in a conditional (unless you mean it to be a wordy way to create unreachable code) is never something you want to do. [1] http://blog.sethladd.com/2012/02/booleans-in-dart.html http://blog.sethladd.com/2012/02/booleans-in-dart.html "The only value that is true is the boolean value true. [...] In a boolean context, everything that is not true is converted to false."
spankalee 12y ago
It's subtle. The conversion rule states that only `true` is true, `null` is an error, and everything else is false. However, the type annotation of `bool` means that in checked mode any non-bool value will cause a type error to be thrown. The reason it's defined this way is for efficient compilation to JavaScript. Control-flow constructs don't have to perform a type-check, just a null check (which can be left out in cases due to null propagation).

by berns 12y ago

dragonwriter 12y ago

Reading your quote from the ECMA standard, "no implicit conversions are supported" may be wrong but at the same time not generally misleading. That is, that function errors on null values and returns false for anything that isn't identical to true, so while it does provide a conversion to boolean from any non-null value, anything that isn't exactly a boolean "true" (unless I misunderstand what "identical" does) is going to return false on conversion. This is consistent with other writings on booleans in Dart [1]. So it appears that Dart has implicit boolean conversions, but it converts everything that is not the single "true" value to false, so using an expression that never can evaluate exactly to boolean true in a conditional (unless you mean it to be a wordy way to create unreachable code) is never something you want to do. [1] http://blog.sethladd.com/2012/02/booleans-in-dart.html http://blog.sethladd.com/2012/02/booleans-in-dart.html "The only value that is true is the boolean value true. [...] In a boolean context, everything that is not true is converted to false."

spankalee 12y ago

It's subtle. The conversion rule states that only `true` is true, `null` is an error, and everything else is false. However, the type annotation of `bool` means that in checked mode any non-bool value will cause a type error to be thrown. The reason it's defined this way is for efficient compilation to JavaScript. Control-flow constructs don't have to perform a type-check, just a null check (which can be left out in cases due to null propagation).