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|
Using a PPX Preprocessor
========================
Our calculator is pretty much opaque: we feed it a string, and it displays a
result (on an error message), but we have now way to know what the internal expression looks like.
In this chapter, we're going to use a `ppx` deriver to generate a `pp_expr`
function that can display expressions.
## Prerequisites
Install [ppx_deriving](https://github.com/ocaml-ppx/ppx_deriving) by running:
```sh
opam install ppx_deriving.5.2.1
```
## Create a test
Add a new test in `test/calc.t`:
```console
$ calc --debug-ast -e '2 * sin (pi / 2)'
```
Run `dune runtest` and see the failure.
Run `dune promote` to add the failure to the test file.
Our goal in the rest of the chapter is to change the output of this test.
## Use `ppx_deriving.show`
Add an `[@@deriving show]` attribute on types in `lib/ast.ml`:
```{code-block} ocaml
:emphasize-lines: 5,13
type op =
| Add
| Mul
| Div
[@@deriving show]
type expr =
| Int of int
| Float of float
| Ident of string
| Op of op * expr * expr
| Call of string * expr
[@@deriving show]
```
This will generate `pp_op`, `show_op`, `pp_expr`, and `show_expr` functions.
To do do we need to instruct Dune to use `ppx_deriving.show` as a preprocessor by updating `lib/dune`:
```{code-block} dune
:emphasize-lines: 4-5
(library
(name calc)
(libraries cmdliner)
(preprocess
(pps ppx_deriving.show))
(foreign_stubs
(language c)
(names calc_stubs)))
```
## Add a `--debug-ast` Flag
We have a few edits to make to `lib/cli.ml`.
First, add a new `cmdliner` flag to parse the command-line and pass it to
`repl` and `eval_lb`:
```{code-block} ocaml
:emphasize-lines: 6-8,11-12
let term =
let open Cmdliner.Term.Syntax in
let+ expr_opt =
let open Cmdliner.Arg in
value & opt (some string) None & info [ "e" ]
and+ debug_ast =
let open Cmdliner.Arg in
value & flag & info [ "debug-ast" ]
in
match expr_opt with
| Some s -> eval_lb ~debug_ast (Lexing.from_string s)
| None -> repl ~debug_ast
```
Then, forward it from `repl` to `eval_lb`:
```{code-block} ocaml
:emphasize-lines: 1,5
let repl ~debug_ast =
while true do
Printf.printf ">> %!";
let lb = Lexing.from_channel Stdlib.stdin in
eval_lb ~debug_ast lb
done
```
Finally, update `eval_lb` to use it:
```{code-block} ocaml
:emphasize-lines: 1,4
let eval_lb ~debug_ast lb =
try
let expr = Parser.main Lexer.token lb in
if debug_ast then Format.eprintf "[debug] %a\n" Ast.pp_exp expr;
let v = eval expr in
Printf.printf "%s\n" (value_to_string v)
with Parser.Error ->
Printf.printf "parse error near character %d" lb.lex_curr_pos
```
Run the tests again with `dune runtest`.
At that point, the output should be correct.
Call `dune promote` to update the expected output.
::::{dropdown} Checkpoint
:icon: location
This is how the project looks like at the end of this chapter.
:::{literalinclude} introduction/dune-project
:caption: dune-project (unchanged)
:language: dune
:::
:::{literalinclude} structure/bin/dune
:caption: bin/dune (unchanged)
:language: dune
:::
:::{literalinclude} structure/bin/calc.ml
:caption: bin/calc.ml (unchanged)
:language: ocaml
:::
:::{literalinclude} using-ppx/lib/dune
:caption: lib/dune
:language: dune
:::
:::{literalinclude} using-ppx/lib/ast.ml
:caption: lib/ast.ml
:language: ocaml
:::
:::{literalinclude} interfacing-with-c/lib/calc_stubs.c
:caption: lib/calc_stubs.c (unchanged)
:language: c
:::
:::{literalinclude} using-ppx/lib/cli.ml
:caption: lib/cli.ml
:language: ocaml
:::
:::{literalinclude} interfacing-with-c/lib/lexer.mll
:caption: lib/lexer.mll
:language: ocaml
:::
:::{literalinclude} development-cycle/lib/parser.mly
:caption: lib/parser.mly (unchanged)
:language: ocaml
:::
:::{literalinclude} structure/test/dune
:caption: test/dune (unchanged)
:language: dune
:::
:::{literalinclude} using-ppx/test/calc.t
:caption: test/calc.t
:language: cram
:::
::::
|
