Why does io\open not work in moonscript?

Question

Moonscript uses \ to call methods so can someone explain to me why the code below does not work:

> file = io\open("mix.exs", "rb")
[string "tmp"]:1: calling 'open' on bad self (string expected, got table)

but when you call it to read the file it does ?

> file\read!
"Code.ensure_loaded?(Hex) and Hex.start

Answer 1

The io.open function expects to get a string as the first argument but io\open (like io:open in lua itself) is actually passing the io table as the first argument. That is it is a method call.

io\open("file", "mode")/io:open("file", "mode") are syntactic sugar for io.open(io, "file", "mode").

This is why file\read! works without an explicit argument because file gets passed as the first argument to the read("file", "format") function.

Answer 2

Moonscript uses \ to call methods

to call member methods. as in a\b c, ... translates to a.b(a,c,...).

this doesn't work here because io.open is a static function (io.open(what,how)), not a member (io.open(self,what,how)).

you couldn't call io:open in Lua either. the only place where io functions allow for being called as members is when you want to read/write stdio.

but when you call it to read the file it does ?

because now it's a member method of the file. you're actually still using io.read there, but the file object has io as a metatable index, therefore allowing you to access the same function via file.read, and since file\read! translates to file.read(file) it's the same thing.

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so essentially the answer boils down to "because io:open doesn't work in Lua".