This is an Erlang parse transform for partial function application, in the spirit of Scheme's SRFI-26.

It enables the use of cuts, as represented by the special variable _, to create anonymous functions with some arguments applied.

For example, to create a function which takes a single integer argument to convert to a hexadecimal string:

> Hex = partial:cut(integer_to_list(_, 16)),
> Hex(255).
"FF"

This library is published to hex.pm as partial. If you're using rebar3 as your build tool, it can be added as a dependency to your rebar.config as follows:

{deps, [{partial}]}.

To use this parse transform, add the following to the top of any module after adding this library to your application:

-compile({parse_transform, partial}).

Parse transforms run as part of module compilation. As such, this can not be used in the shell.

Two special functions are provided, which are used by the parse transform to identify where to apply the transformation. The special cut syntax is only allowed as an argument to these two functions:

• partial:cut
• partial:cute

The two functions are differentiated by when arguments are evaluated. Cut evaluates arguments when the function is called. Cute evaluates arguments only once, when the partial function is constructed.

Cuts are represented by the special variable _. As this variable is usually only legal on the left hand side of a match expression, this use should not conflict with any existing Erlang syntax.

Cuts may be used for function arguments or for the two forms of function names. For instance, all three of these usages have the identical result.

Rev1 = partial:cut(lists:reverse(_)),
Rev1([1, 2, 3]),
Rev2 = partial:cut(lists:_([1, 2, 3])),
Rev2(reverse),
Rev3 = partial:cut(_([1, 2, 3])),
Rev3(fun lists:reverse/1).

All, some or none of the arguments to a function may be cuts.

> lists:map(partial:cut(erlang:'*'(_, 2)), [1, 2, 3]).
[2, 4, 6]

The difference between partial:cut/1 and partial:cute/1 can be seen when dealing with functions with side effects. For instance, when we try and read a value from the process dictionary with get/1.

> Identity = fun (X) -> X end,
> put(example, 1),
> Cut = partial:cut(Identity(get(example))),
> Cute = partial:cute(Identity(get(example))),
> Cut().
1
> Cute().
1
> put(example, 2),
> Cut().
2
> Cute().
1

This also makes partial:cute/1 an easy way to cache expensive computation and reuse it in later calls.

Finally, partial evaluation can make creating pipelines across multiple functions easier:

> lists:foldl(fun (Step, Acc) -> Step(Acc) end,
              "1,2,3,4,5,6",
              [partial:cut(string:split(_, ",", all)),
               partial:cut(lists:map(fun erlang:list_to_integer/1, _)),
               partial:cut(lists:filter(fun (I) -> I rem 2 == 0 end, _))]).
[2, 4, 6]

The parse transform supports a single option: partial_allow_local. This option allows for a bare cut/1 or cute/1 call to be treated the same as the fully qualified partial:cut/1 or partial:cute/1 call.

To enable, either pass as a compiler flag or specify as a compile attribute:

-compile(partial_allow_local).

The transformations are implemented as a replacement of the marker functions with fun expressions. Only direct literal calls to the marker functions are found by the parse transform. Indirect calls, as Module:Function(...) or via erlang:apply/3 are not detected or rewritten and will result in a run time exception. The result of either transform is always a fun expression, even when no unevaluated arguments are found.

The underscore variable _ is used as a placeholder for unevaluated arguments. It is only legal as a standalone expression, as either the function name to call or as an argument. Unevaluated arguments are converted to arguments of the created fun in strict left to right order. There is no support for reordering or duplicating unevaluated arguments.

partial:cut/1 is implemented as a transformation from:

Fun = partial:cut(some_fun(X, Y, _)).

To:

Fun = fun (Arg1) ->
	some_fun(X, Y, Arg1)
end.

partial:cute/1 is implemented as a transformation from:

Fun = partial:cute(some_fun(X, Y, _)).

To:

Fun = (fun () ->
	Arg1 = X,
	Arg2 = Y,
	fun (Arg3) ->
		some_fun(Arg1, Arg2, Arg3)
	end
end)().

Please fork the repo and submit a PR. Tests are run via:

rebar3 eunit

Documentation is autogenerated using edown and edoc via:

rebar3 as markdown edoc

Some effort has been put into using only the erl_syntax (sometimes via merl) interfaces to traverse and modify the parse tree. This was done in the hope of those interfaces being more stable than the raw format, which is explicitly not guaranteed by the documentation. Any PR which depends on erl_parse forms will be asked to rewrite using erl_syntax forms.

The application has only been tested with Erlang/OTP 21 on Windows 10. Reports of success (or failure!) on other versions and operating systems are appreciated.

The most common syntactical element I miss in Erlang is the ability to easily do partial function application. While researching existing solutions in Erlang I came across this erlando issue from 2011, which I thought had some pretty good ideas. Notably, I wanted the ability to use a marker function to make use of the transform explicit and I wanted to severely limit the scope of where cuts could be used in order to simplify reasoning about them. As such, it's reasonable to think of this module as a less powerful version of erlando's cut parse transform.

The datum library also contains a similar parse transform, which was used for reference.

Image by Cuklev

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