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+X-Date: 2024-09-15T03:18:50Z
+X-Note-Id: f53b7c27-df57-4ede-8950-2b55cbb8031f
+Subject: Backtraces in Valeri
+X-Slug: backtraces_in_valeri
+
+At the time of writing this post, I've finished implementing the first version of
+backtraces in [Valeri](https://git.knazarov.com/knazarov/valeri). In case of
+runtime errors, you'll now get the call stack printed to stdout, pretty much
+as you would expect in other programming languages. Let's look at an example:
+
+```
+(fn get-foo (obj)                                                                                                                     (get obj :foo)
+ )
+
+;; This will fail becaue :foo is not present
+;; in the dictionary
+(get-foo {:bar 42})
+```
+
+When executed, this code will result in:
+
+```
+Error KeyError at /home/knazarov/dev/lisp/src/common.cpp:735
+  Function get
+  Function get-foo
+```
+
+The backtrace should look familiar, with the only possible problem being lack
+of source location information except in the last frame. This is because I don't
+yet produce debug information with line number / filename mappings.
+
+What may not be immediately obvious is that `get` is a "native" function that
+is implemented in C++. The reason it appears in the backtrace at all is due
+to how native functions are called. This is not just an FFI call, but such functions
+actually use the normal VM stack, as regular byte-compiled functions do.
+
+This means that if I implement something like `map` or `fold` in C++ and put it into
+the standard library, debugging code that uses them is going to be quite easy, even
+if the C++ routines are nested in the middle of the callstack.
+
+Overall, backtraces and the transition to tree-structured stack were the hardest
+parts of the language so far. Not in terms of the implementation, but just design-wise.
+The new stack structure is going to be used in lots of places starting from exceptions,
+and ending with the coroutines, effect system and the debugger.