Pipe operator |> syntax before 1.0 release

[bnyu]

MoonBit provides a convenient pipe operator |>, which can be used to chain regular function calls:

fn init {
  x |> f // equivalent to f(x)
  x |> f(y) // equivalent to f(x, y)

  // Chain calls at multiple lines
  arg_val
  |> f1 // equivalent to f1(arg_val)
  |> f2(other_args) // equivalent to f2(f1(arg_val), other_args)
}

This is from Moonbit doc, it is cool. But the syntax is not intuitive for multi-parameter functions, because the second argument is written in the first position, this is inconsistent with directly function call. (method call has an implicit receiver parameter which is consistent with function signature, but the pipe syntactic sugar is completely different)

So I suggest we use _ placeholder syntax maybe like this x |> f(_, y) or x |> f($, y)
This also makes we can pipe to second or third parameter not only first:

fn init {
  debug(3.14 |> Double::to_int) // output: 3
  debug(4 |> Array::make(_, 'c')) // output: ['c', 'c', 'c', 'c']
  debug('c' |> Array::make(4, _)) // output: ['c', 'c', 'c', 'c']
}

Originally posted by @bnyu in #299 (comment)