Collection of Erlang parse transforms
Version: 0.2.2-38-gf933a76
Authors: : Serge Aleynikov (saleyn(at)gmail.com).
Collection of Erlang Parse Transforms
License: MIT License
This library includes useful parse transforms including Elixir-like pipeline operator for cascading function calls.
Content
Module | Description |
---|---|
defarg | Support default argument values in Erlang functions |
erlpipe | Elixir-like pipeline operator for Erlang |
listcomp | Fold Comprehension and Indexed List Comprehension |
iif | Ternary if function including iif/3 , iif/4 , ife/3 , ife/4 parse transforms |
str | Stringification 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.
-export([t/2]).
test(A / 10, B / 20) ->
A + B.
The code above is transformed to:
-export([t/2]).
-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) ->
fun7(fun6([1,2,3],
io_lib:format("~p\n", [fun4(Arg3, fun3(mod2:fun2(fun1(Arg1, Arg2))))]),
other_param)).
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:
- https://github.com/stolen/pipeline
- https://github.com/oltarasenko/epipe
- https://github.com/clanchun/epipe
- https://github.com/pouriya/pipeline
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
Example:
[{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)).
Example:
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]),
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.
E.g.:
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
end.
case some_fun(A) of
match -> ok;
nomatch -> error
end.
case L of
[] -> undefined;
false -> undefined;
undefined -> undefined;
_ -> L
end.
case L of
[] -> nil;
false -> nil;
undefined -> nil;
_ -> hd(L)
end.
str
: String transforms
This module implements a transform to stringify an Erlang term.
str(Term)
is equivalent tolists:flatten(io_lib:format("~p", [Term]))
.str(Fmt, Args)
is equivalent tolists:flatten(io_lib:format(Fmt, Args))
.throw(Fmt,Args)
is equivalent tothrow(lists:flatten(io_lib:format(Fmt, Args)))
.
Dowloading
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}]}.