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Finding and fixing errors

Finding errors in your programs is always tricky. Here we’ll cover some common compilation errors and ways to fix them. We’ll also discuss a couple of ways to find and fix runtime errors.

Cannot find variable [name]

This error occurs when a variable cannot be found. The first thing to check is that you’ve spelt the variable name correctly and, if it is an external symbol, that you’ve imported it. Otherwise, there are a couple of things to consider:

> (for i 1 10
.   (print! i))
[ERROR] Cannot find variable i
 => program:[2:11 .. 2:11] ("i")

 2 | (print! i))
   |         ^

Here i cannot be found, despite it is clearly defined. The issue here is that for is missing an argument and so (print! i) is being read as the loop’s step variable. Adding an additional 1 fixes the problem:

> (for i 1 10 1 ;; Note the extra '1' here
.   (print! i))

Previous declaration of [name]

Urn only allows a single definition of a variable in a given scope. When you try to create two variables with the same name, you’ll get an error message. There are a couple of times this could happen.

Firstly, if two modules export variables with the same name, you’ll get a clash.

> (import lua/table ())
. (import lua/os ())
[ERROR] Previous declaration of remove
 => program:[2:1 .. 2:18]
 2 | (import lua/os ())
   | ^^^^^^^^^^^^^^^^^^ imported here
 7 | (define-native remove)
   | ^^^^^^^^^^^^^^^^^^^^^^ new definition here
 5 | (define-native remove)
   | ^^^^^^^^^^^^^^^^^^^^^^ old definition here

As you can see, both lua/table and lua/os define a remove symbol, resulting in clashes. Instead you should either selectively choose just the symbols you need (such as (import lua/table (insert))) or qualify your imports: (import lua/table table).

Runtime errors

Debugging runtime errors in Urn is a tad tricky due to the combination of incorrect line numbers and the dissimilarities between Urn and Lua tracebacks. Generally my process of fixing code is as follows:

  • Compile your Urn to Lua.
  • Find which lines errored and try to map them back to Urn.
  • Add several print! and debug around the erroring code. It might be useful to print out values of variables, as well as what code has and hasn’t been executed.
  • Compile and run again.

We have plans to simplify the first two steps, with the help of line mapping, but that has yet to be implemented.