Example in the doc of Crystal.main does not work

[lugia-kun]

The example described in Crystal.main(Int32, UInt8**) does not work because there is Crystal.main(&).

fun alternative_main(argc : Int32, argv : UInt8**, prc : (Int32, UInt8**) -> Int32) : Int32
  0
end

fun main(argc : Int32, argv : UInt8**) : Int32
  alternative_main(argc, argv, ->Crystal.main)
end 

Showing last frame. Use --error-trace for full trace.

error in line 6
Error: 'Crystal.main' is expected to be invoked with a block, but no block was given

https://play.crystal-lang.org/#/r/9kk9

I have also confirmed this with master:

Using compiled compiler at .build/crystal
Crystal 1.0.0-dev [007dcfc78] (2020-08-19)

LLVM: 10.0.1
Default target: x86_64-unknown-linux-gnu

Workaround

Using wrapped Proc can avoid this error.

fun alternative_main(argc : Int32, argv : UInt8**, prc : (Int32, UInt8**) -> Int32) : Int32
  0
end
 
fun main(argc : Int32, argv : UInt8**) : Int32
  wrap = -> (ac : Int32, av : Pointer(Pointer(UInt8))) { Crystal.main(ac, av) }
  alternative_main(argc, argv, wrap)
end

https://play.crystal-lang.org/#/r/9kkh

[hugopl]

This works.

fun alternative_main(argc : Int32, argv : UInt8**, prc : (Int32, UInt8**) -> Int32) : Int32
  0
end

fun main(argc : Int32, argv : UInt8**) : Int32
  alternative_main(argc, argv, ->Crystal.main(Int32,UInt8**))
end

So I think this just need a minor docs update.