Cut

Introduction

A syntax extension (i.e. parse transform) that adds support for cuts to Erlang. These are inspired by the Scheme form of cuts.

Cuts can be thought of as a light-weight form of abstraction, with similarities to partial application (or currying).

Use

To use this parse-transformer, you must add the necessary -compile attribute to your Erlang source files. For example:

-module(test).
-compile({parse_transform, cut}).
...

Then, when compiling test.erl, you must ensure erlc can locate cut.beam by passing the suitable path to erlc with a -pa or -pz argument. For example:

erlc -Wall +debug_info -I ./include -pa ebin -o ebin  src/cut.erl
erlc -Wall +debug_info -I ./include -pa test/ebin -pa ./ebin -o test/ebin test/src/test.erl

Note: If you're using QLC, you may find you need to be careful as to the placement of the parse-transformer attributes. For example, I've found that -compile({parse_transform, cut}). must occur before -include_lib("stdlib/include/qlc.hrl").

Motivation

The cut parse-transformer is motivated by the frequency with which simple function abstractions are used in Erlang, and the relatively noisy nature of declaring funs. For example, it's quite common to see code like:

with_resource(Resource, Fun) ->
    case lookup_resource(Resource) of
        {ok, R}          -> Fun(R);
        {error, _} = Err -> Err
    end.

my_fun(A, B, C) ->
    with_resource(A, fun (Resource) ->
                         my_resource_modification(Resource, B, C)
                     end).

That is, a fun is created in order to perform variable capture from the surrounding scope but to leave holes for further arguments to be provided. Using a cut, the function my_fun can be rewritten as:

my_fun(A, B, C) ->
    with_resource(A, my_resource_modification(_, B, C)).

Definition

Normally, the variable _ can only occur in patterns: that is, where a match occurs. This can be in assignment, in cases, and in function heads. For example:

{_, bar} = {foo, bar}.

Cut uses _ in expressions to indicate where abstraction should occur. Abstraction from cuts is always performed on the shallowest enclosing expression. For example:

list_to_binary([1, 2, math:pow(2, _)]).

will create the expression

list_to_binary([1, 2, fun (X) -> math:pow(2, X) end]).

and not

fun (X) -> list_to_binary([1, 2, math:pow(2, X)]) end.

It is fine to use multiple cuts in the same expression, and the arguments to the created abstraction will match the order in which the _ var is found in the expression. For example:

assert_sum_3(X, Y, Z, Sum) when X + Y + Z == Sum -> ok;
assert_sum_3(_X, _Y, _Z, _Sum) -> {error, not_sum}.

test() ->
    Equals12 = assert_sum_3(_, _, _, 12),
    ok = Equals12(9, 2, 1).

It is perfectly legal to take cuts of cuts as the abstraction created by the cut is a normal fun expression and thus can be re-cut as necessary:

test() ->
    Equals12 = assert_sum_3(_, _, _, 12),
    Equals5 = Equals12(_, _, 7),
    ok = Equals5(2, 3).

Note that because a simple fun is being constructed by the cut, the arguments are evaluated prior to the cut function. For example:

f1(_, _) -> io:format("in f1~n").

test() ->
    F = f1(io:format("test line 1~n"), _),
    F(io:format("test line 2~n")).

will print out

test line 2
test line 1
in f1

This is because the cut creates fun (X) -> f1(io:format("test line 1~n"), X) end. Thus it is clear that X must be evaluated first, before the fun can be invoked.

Of course, no one would be crazy enough to have side-effects in function argument expressions, so this will never cause any issues!

Cuts are not limited to function calls. They can be used in any expression where they make sense:

Tuples

F = {_, 3},
{a, 3} = F(a).

Lists

dbl_cons(List) -> [_, _ | List].

test() ->
    F = dbl_cons([33]),
    [7, 8, 33] = F(7, 8).

Note that if you nest a list as a list tail in Erlang, it's still treated as one expression. For example:

A = [a, b | [c, d | [e]]]

is exactly the same (right from the Erlang parser onwards) as:

A = [a, b, c, d, e]

That is, those sub-lists, when they're in the tail position, do not form sub-expressions. Thus:

F = [1, _, _, [_], 5 | [6, [_] | [_]]],
%% This is the same as:
%%  [1, _, _, [_], 5, 6, [_], _]
[1, 2, 3, G, 5, 6, H, 8] = F(2, 3, 8),
[4] = G(4),
[7] = H(7).

However, be very clear about the difference between , and |: the tail of a list is only defined following a |. Following a ,, you're just defining another list element.

F = [_, [_]],
%% This is **not** the same as [_, _] or its synonym: [_ | [_]]
[a, G] = F(a),
[b] = G(b).

Records

-record(vector, { x, y, z }).

test() ->
    GetZ = _#vector.z,
    7 = GetZ(#vector { z = 7 }),
    SetX = _#vector{x = _},
    V = #vector{ x = 5, y = 4 } = SetX(#vector{ y = 4 }, 5).

Case

F = case _ of
        N when is_integer(N) -> N + N;
        N                    -> N
    end,
10 = F(5),
ok = F(ok).

See test_cut.erl for more examples, including the use of cuts in list comprehensions and binary construction.

Note that cuts are not allowed where the result of the cut can only be useful by interacting with the evaluation scope. For example:

F = begin _, _, _ end.

This is not allowed, because the arguments to F would have to be evaluated before the invocation of its body, which would then have no effect, as they're already fully evaluated by that point.

License

(The MPL)

Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License.

The Original Code is Erlando.

The Initial Developer of the Original Code is VMware, Inc. Copyright (c) 2011-2013 VMware, Inc. All rights reserved.