Recursion in F#, expected type int but got type "int list
I'm new to F# and want to implement a least common multiple function of a list by doing it recursively, eg lcm(a,b,c) = lcm(a, lcm(b,c)), where lcm of two elements is calculated from the gcd.
I have the following code. I try to match the input of the lcm function with a list of two elements, and otherwise a general list, which I split up into its first element and the remaining part. The part "lcm (tail)" gives a compiler error. It says it was expected to have type "int" but has type "int list -> int". It looks like it says that the expression "lcm tail" is itself a function, which I don't understand. Why is it not an int?
let rec gcd a b =
if b = 0
then abs a
else gcd b (a % b)
let lcmSimple a b = a*b/(gcd a b)
let rec lcm list = function
| [a;b] -> lcmSimple a b
| head::tail -> lcmSimple (head) (lcm (tail))
Best regards.
When defining the function as let f = function | ... let f = function | ... , the argument to the function is implicit, as it is interpreted as let fx = match x with | ... fx = match x with | ... .
Thus let rec lcm list = function |... is a function of two variables, which are list and the implicit variable. This is why the compiler claims that lcm tail is a function - only one variable has been passed, where it expected two. A better version of the code is
let rec gcd a b =
if b = 0
then abs a
else gcd b (a % b)
let lcmSimple a b = a*b/(gcd a b)
let rec lcm = function
| [a;b] -> lcmSimple a b
| head::tail -> lcmSimple (head) (lcm (tail))
| [] -> 1
where the last case has been included to complete the pattern.
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