michalwski / pa

Partial application of Erlang functions

Partial application of Erlang functions

Trivial example

An example says more than a thousand words:

> Times2 = pa:bind(fun erlang:'*'/2, 2).
#Fun<pa.17.35850360>
> lists:map(Times2, [1,2,3]).
[2,4,6]

Real world example

Take an example from MongooseIM (actually a legacy leftover from ejabberd) - this inline fun is 55 lines long!

The fun code has problems with indentation, since the body of the inline fun should be indented more than the outer code and using cases and ifs increases the indentation level even more.

This fun is complex and not easily traceable, because we don't know its real name (at least without referring to erlc -S) - in case of a fun this long, its logic might not be obvious, so tracing and seeing the return value might be really helpful.

A more general example of the same problem looks like this:

do_something_on_a_list_of_items(ListOfItems) ->
    SomeVar1 = get_some_var1(),
    SomeVar2 = get_some_var2(),
    SomeVar3 = get_some_var3(),
    lists:map(fun (Elem) ->
                      %% very long closure using variables
                      %% SomeVar1, SomeVar2, SomeVar3 closed over
                      %% from the outer environment
              end, ListOfItems).

I hope you're convinced that refactoring is necessary.

do_something_on_a_list_of_items(ListOfItems) ->
    SomeVar1 = get_some_var1(),
    SomeVar2 = get_some_var2(),
    SomeVar3 = get_some_var3(),
    lists:map(mk_step(SomeVar1, SomeVar2, SomeVar3), ListOfItems).

mk_step(SomeVar1, SomeVar2, SomeVar3) ->
    fun (Elem) ->
            do_something_with_one_item(SomeVar1, SomeVar2, SomeVar3, Elem)
    end.

do_something_with_one_item(SomeVar1, SomeVar2, SomeVar3, Elem) ->
    %% still a long function using variables
    %% SomeVar1, SomeVar2, SomeVar3
    %% passed in as arguments
    ...

The end result if functionally the same, but do_something_with_one_item/4 is perfectly traceable and doesn't suffer from pathological indentosis.

However, mk_step/3 from above is just boilerplate. The same could be achieved without proliferation of similar mk_sth/3 and mk_sth_else/5 functions throughout the codebase with function partial application. Which Erlang lacks!

Thankfully, this library fixes this nuisance by providing a quite convenient (as far as the syntax permits) implementation:

do_something_on_a_list_of_items(ListOfItems) ->
    SomeVar1 = get_some_var1(),
    SomeVar2 = get_some_var2(),
    SomeVar3 = get_some_var3(),
    lists:map(pa:bind(fun do_something_with_one_item/4,
                      SomeVar1, SomeVar2, SomeVar3), ListOfItems).

do_something_with_one_item(SomeVar1, SomeVar2, SomeVar3, Elem) ->
    %% still a long function using variables
    %% SomeVar1, SomeVar2, SomeVar3
    %% passed in as arguments
    ...

There is more than that!

Of course, map/2 and foreach/2 require unary functions, but foldl/3 a binary one and you might come up with a number of functions with even wilder requirements!

You can define the maximum supported arity by defining the value of the max_partial_arity macro during compilation.

Defining it in your rebar.config file is a pretty convenient way to do so.

The default value is 15.