How to define an function in the eshell (Erlang shell)?

Question

Is there any way to define an Erlang function from within the Erlang shell instead of from an erl file (aka a module)?

Answer 1

Yes but it is painful. Below is a "lambda function declaration" (aka fun in Erlang terms).

1> F=fun(X) -> X+2 end. %%⇒ #Fun <erl_eval.6.13229925> 

Have a look at this post. You can even enter a module's worth of declaration if you ever needed. In other words, yes you can declare functions.

Answer 2

One answer is that the shell only evaluates expressions and function definitions are not expressions, they are forms. In an erl file you define forms not expressions.

All functions exist within modules, and apart from function definitions a module consists of attributes, the more important being the modules name and which functions are exported from it. Only exported functions can be called from other modules. This means that a module must be defined before you can define the functions.

Modules are the unit of compilation in erlang. They are also the basic unit for code handling, i.e. it is whole modules which are loaded into, updated, or deleted from the system. In this respect defining functions separately one-by-one does not fit into the scheme of things.

Also, from a purely practical point of view, compiling a module is so fast that there is very little gain in being able to define functions in the shell.