error-stack-trace.md

Error Reporting and Stack Traces

The mixin recursion limit is configurable. Exceeding the recursion limit will raise an error that includes the stack trace. If the number of frames exceeds the display window, the middle frames will be omitted.

Example 1: Recursion exceeds the limit

recursion.less:

.mixin(@arg: 0) when (@arg < 30) {
    (~'rule-@{arg}') {
        color: #000 + @arg;
    }
    .mixin(@arg + 1);
}
.parent {
    .mixin;
}

Executing the above template with a recursion limit of 20, output:

An error occurred in 'recursion.less':

Line               Statement
----------------------------
recursion.less:9   .parent {
recursion.less:10      .mixin;
recursion.less:6           .mixin(1);
recursion.less:6               .mixin(2);
recursion.less:6                   .mixin(3);
recursion.less:6                       .mixin(4);

.. skipped 9 frames

recursion.less:6                           .mixin(14);
recursion.less:6                               .mixin(15);
recursion.less:6                                   .mixin(16);
recursion.less:6                                       .mixin(17);
recursion.less:6                                           .mixin(18);
recursion.less:6                                               .mixin(19);

ExecuteError MIXIN_RECURSE: Mixin call [.mixin] exceeded the recursion limit of 20

Example 2: Mutual recursion without guard conditions

ping-pong.less:

.foo(@val) {
    .bar(@val + 1);
}
.bar(@val) {
    .foo(@val + 1);
}
.ruleset {
    .foo(1);
}

The mutually-recursing mixins lack guards and would execute forever, but the code is stopped by the default recursion limit:

An error occurred in 'ping-pong.less':

Line              Statement
---------------------------
ping-pong.less:8  .ruleset {
ping-pong.less:9      .foo(1);
ping-pong.less:3          .bar(2);
ping-pong.less:6              .foo(3);
ping-pong.less:3                  .bar(4);
ping-pong.less:6                      .foo(5);

.. skipped 53 frames

ping-pong.less:6                          .foo(59);
ping-pong.less:3                              .bar(60);
ping-pong.less:6                                  .foo(61);
ping-pong.less:3                                      .bar(62);
ping-pong.less:6                                          .foo(63);
ping-pong.less:3                                              .bar(64);

ExecuteError MIXIN_RECURSE: Mixin call [.foo] exceeded the recursion limit of 64