Has "let" syntax changed recently? And why are the REPL and compiler behaving differently?

[amphibulous]

@yawaramin

Pretty much any language with compile-time syntax checking will treat an expression as a syntax error if it is placed outside of expression context.

Yes: all languages do indeed have syntax. The point is that no other language community has docs explaining a key element of syntax so badly. None. And I’ve written elisp, so I don’t say this lightly.

Also… You seem to use top level/toplevel - and gawd help us, top-level - to mean both the REPL and the outermost scope of definitions. Hasn’t anyone ever suspected that this would lead to confusion???

Just take a look at results thrown up googling “single vs double semicolon”:

..a stack load of links removed because they’re not allowed..

A lot of people in these discussions say that ;; is REPL only - then others prove that it isn’t. Even people writing ocaml don’t seem to understand some of the basic rules. This is not a dignified way to program: its coding through ritual, not understanding. In a language that is supposed to be about clarity.

Eg

Double semicolon peculiarity

That value isn’t seen interactively – the examples given immediately after that part of the text are all of files. I doubt the value myself; the effect for someone learning OCaml is that inserting ;; makes syntax errors go away: let () = print_endline "hi" print_endline "there" let s = "!" in print_endline s first error: “Syntax error” on line 5, at the in. A mistake with let ... in ... is always the worst, as far as OCaml error messages go. let () = print_endline "hi" print_endline "there…

the toplevel reads in lines until it sees a ;; , then it ignores everything after that and tries to interprets everything before it as valid OCaml code. The ‘comments’ that you had after ;; were just ignored.

That value isn’t seen interactively – the examples given immediately after that part of the text are all of files. I doubt the value myself; the effect for someone learning OCaml is that inserting ;; makes syntax errors go away:

This is a mess. Much worse, it’s a poorly explained mess that will blow in the face of anyone doing early experiments with the language. The closest thing I’ve seen to a decent explanation is what the OP in that thread says he concluded from experiment:

I came up with these rules to follow that fixed my wanting to use ;; in source files:

1. don’t use “let … in” in toplevel code
2. write “let () =\n do_this ();\n do_that ()” instead of
   “do_this (); do_that ()” in toplevel code
3. treat ; like an infix operator rather than a terminator or an
   optional separator. Type it in expectation of a right-hand-side.

‘toplevel’ meaning, not as part of the body of some other expression.

…But people shouldn’t have to code that way. Key language rules should be clearly stated. And from that same discussion

lpw25 > ;; is a valid part of the syntax – but is an unnecessary one and it is not supported in the REPL. ;; should be read as a prefix meaning “the following is a item-level construct”.

But

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Double semi-colon isn’t needed in the language proper. It’s used to “submit” a batch of lines to the REPL

..So the people coding in ocaml can’t agree on something this basic. And worse, I just tested whether ;; works in the REPL and it does - and the person saying it doesn’t is listed as a “Maintainer”..!?

Again - this is a mess. In fact it’s a mess like nothing I’ve ever seen before - and google shows that this has been a source of confusion for years. Ocaml should be a trivial language for me to pick up - I’ve written lisp, Scala, Erlang and even some Haskell. Instead I’ve had to detour to investigate this incredibly trivial point and discovered a world of confusion among people using the language.