Erlang getting error ** 1: syntax error before: '->' **

Question

Hi,

I have started some hands on in Erlang and I am getting : ** 1: syntax error before: '->' ** whenever i am declaring any function for eg. to calculate sum of a list (this is experimental, of cource there is Built In Function for find sum of a list).

sum([]) -> 0;
sum([H | T]) -> H + sum(T).

in erl shell (v 5.5.5).

Thanks in advance

Answer 1

Hi,

You can't define functions in the shell using the same syntax as in an erl file.

You can define fun's though.

Syntax in the shell needs to be:

Sum = fun([], _) -> 0; ([H | T], F) -> H + F(T, F) end,
Sum([1,2,3], Sum).

Note that recursive anonymous functions (which this is) are defined in an ugly way. You basically have to pass the function as an argument to itself.

Answer 2

The straight answer is that in a module definition file you have attributes, like -module()., -export(). etc, and function definitions, while in the shell you enter expressions to be evaluated. A function definition is not an expression.

If you want to define a local, temporary function in the shell you need to use fun's as @Daniel has shown. These are really anonymous unnamed functions so calling themselves recursively is a pain, which is not specific to Erlang but common to all anonymous functions.

N.B.

Sum = fun([], _) -> 0; ([H | T], F) -> H + F(T, F) end.

in shell does NOT define a function called Sum but defines an anonymous function and binds the variable Sum to it.

This is also why the only thing you can do in a module is define functions and not expressions to be evaluated when the module is loaded.

Answer 3

Or use the lists:foldl/2 function. This is copied directly from the Erlang Reference Manual.

1> lists:foldl(fun(X, Sum) -> X + Sum end, 0, [1,2,3,4,5]).
15