Collection of Erlang parse transforms

Version: 0.2.2-77-gb2e2b65

Authors: : Serge Aleynikov (saleyn(at)

Collection of Erlang Parse Transforms

License: MIT License

This library includes useful parse transforms including Elixir-like pipeline operator for cascading function calls.


defargSupport default argument values in Erlang functions
erlpipeElixir-like pipe operator for Erlang
listcompFold Comprehension and Indexed List Comprehension
iifTernary if function including iif/3, iif/4, nvl/2, nvl/3 parse transforms
strStringification functions including str/1, str/2, and throw/2 parse transforms

defarg: Support default argument values in Erlang functions

Presently the Erlang syntax doesn't allow function arguments to have default parameters. Consequently a developer needs to replicate the function definition multiple times passing constant defaults to some parameters of functions.

This parse transform addresses this shortcoming by extending the syntax of function definitions at the top level in a module to have a default expression such that for A / Default argument the Default will be used if the function is called in code without that argument.

Though it might seem more intuitive for programmers coming from other languages to use the assignment operator = for defining default arguments, using that operator would change the current meaning of pattern matching of arguments in function calls (i.e. test(A=10) is presently a valid expression). Therefore we chose the / operator for declaring default arguments because it has no valid meaning when applied in declaration of function arguments, and presently without the defarg transform, using this operator (e.g. test(A / 10)) would result in a syntax error detected by the compiler.

test(A / 10, B / 20) ->
  A + B.

The code above is transformed to:

-export([t/0, t/1]).
test()    -> test(10);
test(A)   -> test(A, 20);
test(A,B) -> A+B.

The arguments with default values must be at the end of the argument list:

test(A, B, C / 1) ->    %% This is valid
test(A / 1, B, C) ->    %% This is invalid

NOTE: The default arguments should be constant expressions. Function calls in default arguments are not supported!

test(A / erlang:timestamp()) ->     %% !!! Bad syntax

erlpipe: Erlang Pipe Operator

Inspired by the Elixir's |> pipeline operator. This transform makes code with cascading function calls much more readable by using the / as the pipeline operator. In the LHS / RHS / ... Last. notation, the result of evaluation of the LHS expression is passed as an argument to the RHS expression. This process continues until the Last expression is evaluated. The head element of the pipeline must be either a term to which the arithmetic division / operator cannot apply (i.e. not integers, floats, variables, functions), or if you need to pass an integer, float, variable, or a result of a function call, wrap it in a list brackets.

It transforms code from:

print(L) when is_list(L) ->
  [3, L]                                         %% Multiple items in a list become arguments to the first function
  / lists:split                                  %% In Module:Function calls parenthesis are optional
  / element(1, _)                                %% '_' is the placeholder for the return value of a previous call
  / binary_to_list
  / io:format("~s\n", [_]).
test1(Arg1, Arg2, Arg3) ->
  [Arg1, Arg2]                                   %% Arguments must be enclosed in `[...]`
  / fun1                                         %% In function calls parenthesis are optional
  / mod:fun2
  / fun3()
  / fun4(Arg3, _)                                %% '_' is the placeholder for the return value of a previous call
  / fun ff/1                                     %% Inplace function references are supported
  / fun erlang:length/1                          %% Inplace Mod:Fun/Arity function references are supported
  / fun(I) -> I end                              %% This lambda will be evaluated as: (fun(I) -> I end)(_)
  / io_lib:format("~p\n", [_])
  / fun6([1,2,3], _, other_param)
  / fun7.
test2() ->
  % Result = Argument   / Function
  3        = abc        / atom_to_list / length, %% Atoms    can be passed to '/' as is
  3        = "abc"      / length,                %% Strings  can be passed to '/' as is
  "abc"    = <<"abc">>  / binary_to_list,        %% Binaries can be passed to '/' as is
  "1,2,3"  = {$1,$2,$3} / tuple_to_list          %% Tuples   can be passed to '/' as is
                        / [[I] || I <- _]        %% The '_' placeholder is replaced by the return of tuple_to_list/1
                        / string:join(","),      %% Here a call to string:join/2 is made
  "1"      = [min(1,2)] / integer_to_list,       %% Function calls, integer and float value
  "1"      = [1]        / integer_to_list,       %% arguments must be enclosed in a list.
  "1.0"    = [1.0]      / float_to_list([{decimals,1}]),
  "abc\n"  = "abc"      / (_ ++ "\n"),           %% Can use operators on the right hand side
  2.0      = 4.0        / max(1.0, 2.0),         %% Expressions with lhs floats are unmodified
  2        = 4          / max(1, 2).             %% Expressions with lhs integers are unmodified
test3() ->
  A   = 10,
  B   = 5,
  2   = A / B,                                   %% LHS variables (e.g. A) are not affected by the transform
  2.0 = 10 / 5,                                  %% Arithmetic division for integers, floats, variables is unmodified
  2.0 = A / 5,                                   %% (ditto)
  5   = max(A,B) / 2.                            %% Use of division on LHS function calls is unaffected by the transform

to the following equivalent:

test1(Arg1, Arg2, Arg3) ->
            io_lib:format("~p\n", [(fun(I) -> I end)(
                                      erlang:length(ff(fun4(Arg3, fun3(mod2:fun2(fun1(Arg1, Arg2)))))))]),
print(L) when is_list(L) ->
  io:format("~s\n", [binary_to_list(element(1, lists:split(3, L)))]).
test2() ->
  3       = length(atom_to_list(abc)),
  3       = length("abc"),
  "abc"   = binary_to_list(<<"abc">>),
  "1,2,3" = string:join([[I] || I <- tuple_to_list({$1,$2,$3})], ","),
  "1"     = integer_to_list(min(1,2)),
  "1"     = integer_to_list(1),
  "1.0"   = float_to_list(1.0, [{decimals,1}]),
  "abc\n" = "abc" ++ "\n",
  2.0     = 4.0 / max(1.0, 2.0),
  2       = 4   / max(1, 2).

Similarly to Elixir, a special tap/2 function is implemented, which passes the given argument to an anonymous function, returning the argument itself. The following:

f(A) -> A+1.
test_tap() ->
  [10] / tap(f)
       / tap(fun f/1)
       / tap(fun(I) -> I+1 end).

is equivalent to:

test_tap() ->
        (fun(I) -> I+1 end)(10),

Some attempts to tackle this pipeline transform have been done by other developers:

Yet, we subjectively believe that the choice of syntax in this implementation of transform is more succinct and elegant, and doesn't attempt to modify the meaning of the / operator for arithmetic LHS types (i.e. integers, floats, variables, and function calls).

Why didn't we use |> operator instead of / to make it equivalent to Elixir? Parse transforms are applied only after the Erlang source code gets parsed to the AST representation, which must be in valid Erlang syntax. The |> operator is not known to the Erlang parser, and therefore, using it would result in the compile-time error. We had to select an operator that the Erlang parser would be happy with, and / was our choice because visually it resembles the pipe | character more than the other operators.

listcomp: Fold and Indexed List Comprehensions

Indexed List Comprehension

Occasionally the body of a list comprehension needs to know the index of the current item in the fold. Consider this example:

[{1,10}, {2,20}] = element(1, lists:foldmapl(fun(I, N) -> {{N, I}, N+1} end, 1, [10,20])).

Here the N variable is tracking the index of the current item I in the list. While the same result in this specific case can be achieved with lists:zip(lists:seq(1,2), [10,20]), in a more general case, there is no way to have an item counter propagated with the current list comprehension syntax.

The Indexed List Comprehension accomplishes just that through the use of an unassigned variable immediately to the right of the || operator:

  [{Idx, I} || Idx, I <- L].
%              ^^^
%               |
%               +--- This variable becomes the index counter


[{1,10}, {2,20}] = [{Idx, I} || Idx, I <- [10,20]].

Fold Comprehension

To invoke the fold comprehension transform include the initial state assignment into a list comprehension:

  [S+I || S = 1, I <- L].
%  ^^^    ^^^^^
%   |       |
%   |       +--- State variable bound to the initial value
%   +----------- The body of the foldl function

In this example the S variable gets assigned the initial state 1, and the S+I expression represents the body of the fold function that is passed the iteration variable I and the state variable S:

lists:foldl(fun(I, S) -> S+I end, 1, L).

A fold comprehension can be combined with the indexed list comprehension by using this syntax:

  [do(Idx, S+I) || Idx, S = 10, I <- L].
%  ^^^^^^^^^^^^    ^^^  ^^^^^^
%       |           |     |
%       |           |     +--- State variable bound to the initial value (e.g. 10)
%       |           +--------- The index variable bound to the initial value of 1
%       +--------------------- The body of the foldl function can use Idx and S

This code is transformed to:

element(2, lists:foldl(fun(I, {Idx, S}) -> {Idx+1, do(Idx, S+I)} end, {1, 10}, L)).


33 = [S + Idx*I || Idx, S = 1, I <- [10,20]],
30 = [print(Idx, I, S) || Idx, S=0, I <- [10,20]].
% Prints:
%   Item#1 running sum: 10
%   Item#2 running sum: 30
print(Idx, I, S) ->
  Res = S+I,
  io:format("Item#~w running sum: ~w\n", [Idx, Res]),

iif: Ternary and quaternary if

This transform improves the code readability for cases that involve simple conditional if/then/else tests in the form iif(Condition, Then, Else). Since this is a parse transform, the Then and Else expressions are evaluated only if the Condition evaluates to true or false respectively.


iif(tuple_size(T) == 3, good, bad).       %% Ternary if
iif(some_fun(A), match, ok, error).       %% Quaternary if
nvl(L, undefined).
nvl(L, nil, hd(L))

are transformed to:

case tuple_size(T) == 3 of
  true      -> good;
  _         -> bad
case some_fun(A) of
  match     -> ok;
  nomatch   -> error
case L of
  []        -> undefined;
  false     -> undefined;
  undefined -> undefined;
  _         -> L
case L of
  []        -> nil;
  false     -> nil;
  undefined -> nil;
  _         -> hd(L)

str: String transforms

This module implements a transform to stringify an Erlang term.

Two other shorthand transforms are optionally supported:


erlc +debug_info -Dstr_b2l -Dstr_i2l +'{parse_transform, str}' -o ebin your_module.erl


Building and Using

$ make

To use the transforms, compile your module with the +'{parse_transform, Module}' command-line option, or include -compile({parse_transform, Module}). in your source code, where Module is one of the transform modules implemented in this project.

To use all transforms implemented by the etran application, compile your module with this command-line option: +'{parse_transform, etran}'.

erlc +debug_info +'{parse_transform, etran}' -o ebin your_module.erl

If you are using rebar3 to build your project, then add to rebar.config:

{deps, [{etran, ".*", {git, "git://", "master"}}]}.
{erl_opts, [debug_info, {parse_transform, etran}]}.