Syntax Error In OCaml Multiply Function

Question

I wrote a function that is giving me a syntax error, not exactly sure why. You can skip to the bottom of this and read the code and error, the rest of the stuff in here is just more information that might not be needed.

I have to make a multiply numbers function that multiplies two lists of numbers and returns the result as a list.

let bigMul l1 l2 = 
let f a x = failwith "to be implemented" in
let base = failwith "to be implemented" in
let args = failwith "to be implemented" in
let (_, res) = List.fold_left f base args in
res

It multiplies by pretending representing big ints as lists so 1234x24 is [1;2;3;4] x[2;4] It uses several functions that I already wrote and tested. One is mulByDigit which multiplies each int in a list by an int and returns a list such as [2;3;4;5] 1 would return [2;3;4;5]. It also uses padZero which takes 2 lists of ints and makes them equal in length by adding zeroes to the shorter one and returns a tuple with both lists ex [1;2;3] and [1] would return a tuple with ([1;2;3],[0;0;1]). The last function bigAdd takes 2 lists and adds them and returns the result so like for [1;2;3] [1;2;3] it would give [2;4;6]. All of those functions have been tested and work correctly so Im not gonna provide the code for them.

I wrote a function which follows the logic of taking list l1, multiplying it by every digit of l2 with trailing zeroes depending on the digit and maintaining the sum. Ex if list 1 is [1;2;3;4] and l2 is [2;4] i take the second list and reverse it first of all to get [4;2] and then i multiply 1234 by 4 and get the result. I add this result to the current value in the accumulator which is nothing at first and increment the i to let me know to have a trailing zero for the next one. Then i take the 2 remaining and multiply it by 1234 and have a trailing zero so 24680. I add this to my old accumulator to get the final value and return the value when there are no more numbers left in l2. So 1234+24680=25914. Here is the function that I wrote. a is the accumulator in it, i is for keeping track of how many zeroes I need

let bigMul l1 l2 = 
  let f (i,a) x =
     let sum = padZero ( (mulByDigit x l1)@(clone 0 i) ) a in
     in let (first,second) = match sum with 
        | (y,z)->(y,z) in

            ( i+1, ( bigAdd first second ) ) in

  let base =  (0,[]) in
  let args =  List.rev ( l2 )  in
  let (_, res) = List.fold_left f base args in
    res

im getting a syntax error when i run it that says unbound value l2. Not sure why so any thoughts.

functions that i wrote already I dont want to post them in here because that would be posting almost an entire homework assignment in here. Ill give their prototypes though. — Anatoliy Sokolov
padZero [9;9] [1;0;0;2];; - : int list * int list = ([0;0;9;9],[1;0;0;2]) — Anatoliy Sokolov
You really should get merlin, it can help you type check your programs earlier and spot the mistakes. — Edgar Aroutiounian

Jeffrey Scofield Jeffrey Scofield · Accepted Answer · 2015-10-27T04:14:14

Here's what I see:

 let sum = padZero ( (mulByDigit x l1)@(clone 0 i) ) a in
 in let (first,second) = match sum with

There are two in keywords in a row there.

For what it's worth, this:

let (first,second) = match sum with 
| (y,z)->(y,z)
in

Is equivalent to:

let (first, second) = sum in

Syntax Error In OCaml Multiply Function

2 Answers