windows - Recursive function call hanging, Erlang -

i teaching self erlang. going until found problem function.

-module(chapter). -compile(export_all).  list_length([])      ->   0; list_length([_|xs])  ->   1+list_length([xs]). 

this taken out of textbook. when run code using otp 17, hangs, meaning sits shown below.

1> c(chapter). {ok,chapter} 2> chapter:list_length([]). 0 3> chapter:list_length([1,2]). 

when looking in task manager erlang otp using 200 mb 330 mb of memory. causes this.

it not terminating because creating new non-empty list in every case: [anything] non-empty list, if list contains empty list member ([[]] non-empty list of 1 member).

a proper list terminates this: [ | [] ].

so in mind...

list_length([])      ->   0; list_length([_|xs])  ->   1 + list_length(xs). 

in functional languages "proper lists" cons-style lists. check out the wikipedia entry on "cons" , the erlang documentation lists, , meditate on examples of list operations see written in example code bit.

notes

1. putting whitespace around operators thing; prevent doing confused stuff arrows , binary syntax operators next each other along avoiding few other ambiguities (and easier read anyway).

2. as steve points out, memory explosion noticed because while function recursive, not tail recursive -- is, 1 + list_length(xs) leaves pending work done, must leave reference on stack. have add 1 must complete execution of list_length/1, return value, , in case remember pending value many times there members in list. read steve's answer example of how tail recursive functions can written using accumulator value.