syntax error while loop in OCaml

Question

What's the problem with the code below? I'am struggling a lot with knowing when to use ';' or ';;' or use begin end in OCaml. Here i need to read some edges and insert into graph but i need to link this with rest of program so that it can use g(a graph) with all the edges. When i do this it says Error: Syntax error

let i = ref n in
while !i > 0 do
( 
    let pair = read_edge Scanning.stdin in
    let g = insert_edge (fst pair) (snd pair) g in
    i := !i - 1 
)
done in (* giving error in this line *)

let rec do_stuff l = 
  match l with
  | [] -> ()
  | h::t -> print_int h;do_stuff t in

( * more functions)

sepp2k · Accepted Answer · 2016-04-28 12:58:46Z

3

in is part of the syntax of let (it's "let" pattern "in" expression). The syntax of a while loop is simply "while" expression "do" expression "done", so there's no in in there.

To make your code compile you can replace in with a ;, so it executes the while loop followed by the let expression after it. However it seems strange to me that you'd have a while loop between a bunch of function definitions.

Also note that in the loop you do let g = ..., but then you're not using g anywhere in the let's body. So that binding accomplishes nothing.

answered Apr 28, 2016 at 12:58

sepp2k

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5 Comments

Richard-Degenne Over a year ago

Actually, <expr>; is only syntactic sugar for let () = <expr> in if I'm not mistaken.

power_output Over a year ago

My idea of let g = .... was to keep updating g so that when while loop finishes it can be used by the rest of the program. I wanted to link it with done in, if i can't do it how can i solve this? I think i saw done in before but now that i think of it it was in a for loop not in a while loop.

sepp2k Over a year ago

@power_output The only way you'd see done in would be if the loop was used as the value in a let expression, i.e. let something = while cond do ... done in .... Anyway, let creates a new variable - it does not affect the value of an existing variable (which you can't do). If you want to be able to reassign something, you need a ref (or anything with a mutable member).

power_output Over a year ago

Can you tell me what is usual structure of OCaml program? I want to do dfs on a graph, reading input from stdin. Should i declare all the functions that i will use first and then at the end start reading from stdin to finally call a function that will start everything? I'am going to have to read lines from stdin in a while loop, each line at some point has the edge to insert which mean graph is updated with each line. If i use let g = add_new_edge ... i'am creating variables each time should i then use ref? I feel like it's bad practice in OCaml but i don't know better.

sepp2k Over a year ago

@power_output "Should i declare all the functions that i will use first and then at the end start reading from stdin to finally call a function that will start everything?" Yes, that sounds like the proper way to do it. As for updating the graph, you should probably use a recursive function rather than a while loop.

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