Collection of Erlang parse transforms

Version: 0.2.2-38-gf933a76

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 pipeline operator for Erlang
listcompFold Comprehension and Indexed List Comprehension
iifTernary if function including iif/3, iif/4, ife/3, ife/4 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 theDefault' will be used if the function is called in code without that argument.

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, functions), or if you need to pass integer(s) or float(s), wrap them in a list brackets.

It transforms code from:

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 function
  / io_lib:format("~p\n", [_])
  / fun6([1,2,3], _, other_param)
  / fun7.
print(L) when is_list(L) ->
  [3, L]                                        %% Multiple items in a list are passed as arguments to the first function
  / lists:split
  / element(1, _)
  / binary_to_list
  / io:format("~s\n", [_]).
test2() ->
  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 <- _]
                       / string:join(_, ","),
  "1"     = [min(1,2)] / integer_to_list,       %% Function calls, integer and float values must be passed as a list
  "1"     = [1]        / integer_to_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

to the following equivalent:

test1(Arg1, Arg2, Arg3) ->
            io_lib:format("~p\n", [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).

Similar 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 and floats).

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 comprehension that returns a non-tuple expression:

  [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 = 0, 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 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).
iif(some_fun(A), match, ok, error).
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.


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 YourModule.erl

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

{erl_opts, [debug_info, {parse_transform, etran}]}.