main can take up to two arguments

[peter-ahe-google]

From https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.ey3rvr2kjush:

"Then, the top level function main that is in the exported namespace of S is invoked with no arguments. It is a run time error if S does not declare or import a top level function main."

However, dart2js and the VM agree that the method can be passed up to two arguments. I'm not sure exactly what they are, which is why I looked at the specification.

[peter-ahe-google]

I suggest a rule along these lines:

Given these typedefs:

typedef ZeroParameters();

typedef OneParameter(a);

If main is a top-level function and a subtype of OneParameter, it is invoked with one argument whose runtime type is a subtype of List<String>.

Otherwise, if main is a top-level function and a subtype of ZeroParameters, it is invoked with no arguments.

Otherwise, it is a static (type?) warning and a runtime error.

This leads to this test case:

main(

a /// 01: ok
a, b /// 02: static type warning, runtime error
a, b, c /// 03: static type warning, runtime error
a, b, {c} /// 04: static type warning, runtime error
a, b, [c] /// 05: static type warning, runtime error

[a] /// 20: ok
a, [b] /// 21: ok
[a, b] /// 22: ok

{a} /// 41: ok
a, {b} /// 42: ok
{a, b} /// 43: ok
[a, b, c] /// 44: ok
{a, b, c} /// 45: ok

) {
}

Right now, dart2js fails most of these cases (it reports compile time errors).
The VM fails cases 02-05.

I have a CL which makes dart2js behave as described above.

---

cc @larsbak.
cc @iposva-google.
cc @bwilkerson.

[peter-ahe-google]

Update: the VM doesn't fail 03.

[peter-ahe-google]

cc @johnniwinther.

[gbracha]

Ok, the latest spec has a fix for this.

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Added Done label.

[peter-ahe-google]

I took a look at the latest spec, it says:

"First, $S$ is compiled as a library as specified above. Then, the top-level function \code{main} that is in the exported namespace of $S$ is invoked. If \code{main} has no formal parameters, it is invoked with no arguments. Otherwise \code{main} is invoked with a single actual argument whose type implements \code{List$<$String$>$}. It is a run time error if $S$ does not declare or import a top-level function \code{main}. It is a static warning if \code{main} has more than one required parameter."

That seems to match what I said in comment #­1.

Since then I have learned why dart2js and the VM accepts a second parameter to main.

This is used from Isolate.spawnUri https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-isolate.Isolate#id_spawnUri

My comment #­1 can be extended to handle the second parameter:

Given these typedefs:

typedef ZeroParameters();

typedef OneParameter(a);

typedef TwoPwarmeters(a,b);

If main is a top-level function and a subtype of TwoParameter, it is invoked with two arguments, the first argument's runtime type is a subtype of List<String>, the second argument depends on the environment.

Rationale: see Isolate.spawnUri.

Otherwise, if main is a top-level function and a subtype of OneParameter, it is invoked with one argument whose runtime type is a subtype of List<String>.

Otherwise, if main is a top-level function and a subtype of ZeroParameters, it is invoked with no arguments.

Otherwise, it is a static (type?) warning and a runtime error.

---

cc @lrhn.
cc @vsmenon.
cc @sigmundch.
Added Triaged label.

[peter-ahe-google]

Marked this as blocking #5683.

[gbracha]

Ok, I've revised the spec to account for the second arguments as well. See

https://codereview.chromium.org/280473004/

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Added Accepted label.

[peter-ahe-google]

Unmarked this as blocking #5683.

[gbracha]

Added Done label.