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Update 1wf_01_debugging.md #2897

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27 changes: 27 additions & 0 deletions data/tutorials/guides/1wf_01_debugging.md
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Expand Up @@ -161,6 +161,33 @@ purpose formats are more suited to get the relevant information, than what can
be output automatically by the generic pretty-printer used by the trace
mechanism.

Print debugging is assisted by compiler builtins that allow a printed error
message to refer to the program location that the error was raised from.
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Print debugging is assisted by compiler builtins that allow a printed error
message to refer to the program location that the error was raised from.
Compiler builtins improve debugging messages. The location of the error in the program can be displayed.

For example,
```ocaml
match Message.unpack response with
| Some y -> y
| None -> Printf.printf "Invalid message at %s" __LOC__; flush stdout;
raise Invalid_argument
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| None -> Printf.printf "Invalid message at %s" __LOC__; flush stdout;
raise Invalid_argument
| None -> (Printf.eprintf "Invalid message at %s" __LOC__; raise Invalid_argument)

```
The compiler builtin `__LOC__` is substituted at compile time with the
location it occurs in the program, described as a string
"File %S, line %d, characters %d-%d".
One can also get the file name, line number, start character and
end character directly through the __POS__ builtin:
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The compiler builtin `__LOC__` is substituted at compile time with the
location it occurs in the program, described as a string
"File %S, line %d, characters %d-%d".
One can also get the file name, line number, start character and
end character directly through the __POS__ builtin:
At compile time, the `__LOC__` builtin is substituted with its location in the program, described as a string `"File %S, line %d, characters %d-%d"`. One can also get the file name, line number, start character and end character directly through the `__POS__` builtin:

```ocaml
match Message.unpack response with
| Some y -> y
| None ->
let fname, lnum, _cstart, _cend = __POS__ in
Printf.printf "At line %d in file %s, an incorrect response was passed to Message.unpack"
lnum fname;
flush stdout; raise Invalid_argument
```

Documentation for all debugging compiler builtins is in the
[standard library documentation](https://ocaml.org/manual/5.2/api/Stdlib.html#1_Debugging).
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Documentation for all debugging compiler builtins is in the
[standard library documentation](https://ocaml.org/manual/5.2/api/Stdlib.html#1_Debugging).
Compiler builtins are described in the
[standard library](https://ocaml.org/manual/5.2/api/Stdlib.html#1_Debugging) documentation.


## The OCaml Debugger

We now give a quick tutorial for the OCaml debugger (`ocamldebug`). Before
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