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author | Robin Krahl <robin.krahl@ireas.org> | 2020-01-07 11:18:04 +0000 |
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committer | Daniel Mueller <deso@posteo.net> | 2020-01-08 09:20:25 -0800 |
commit | 5e20a29b4fdc8a2d442d1093681b396dcb4b816b (patch) | |
tree | 55ab083fa8999d2ccbb5e921c1ffe52560dca152 /syn/examples/trace-var/README.md | |
parent | 203e691f46d591a2cc8acdfd850fa9f5b0fb8a98 (diff) | |
download | nitrocli-5e20a29b4fdc8a2d442d1093681b396dcb4b816b.tar.gz nitrocli-5e20a29b4fdc8a2d442d1093681b396dcb4b816b.tar.bz2 |
Add structopt dependency in version 0.3.7
This patch series replaces argparse with structopt in the argument
handling code. As a first step, we need structopt as a dependency.
Import subrepo structopt/:structopt at efbdda4753592e27bc430fb01f7b9650b2f3174d
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Import subrepo rustversion/:rustversion at 0c5663313516263059ce9059ef81fc7a1cf655ca
Import subrepo syn-mid/:syn-mid at 5d3d85414a9e6674e1857ec22a87b96e04a6851a
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Diffstat (limited to 'syn/examples/trace-var/README.md')
-rw-r--r-- | syn/examples/trace-var/README.md | 61 |
1 files changed, 61 insertions, 0 deletions
diff --git a/syn/examples/trace-var/README.md b/syn/examples/trace-var/README.md new file mode 100644 index 0000000..b93fae2 --- /dev/null +++ b/syn/examples/trace-var/README.md @@ -0,0 +1,61 @@ +An example of an attribute procedural macro. The `#[trace_var(...)]` attribute +prints the value of the given variables each time they are reassigned. + +- [`trace-var/src/lib.rs`](trace-var/src/lib.rs) +- [`example/src/main.rs`](example/src/main.rs) + +Consider the following factorial implementation. + +```rust +#[trace_var(p, n)] +fn factorial(mut n: u64) -> u64 { + let mut p = 1; + while n > 1 { + p *= n; + n -= 1; + } + p +} +``` + +Invoking this with `factorial(8)` prints all the values of `p` and `n` during +the execution of the function. + +``` +p = 1 +p = 8 +n = 7 +p = 56 +n = 6 +p = 336 +n = 5 +p = 1680 +n = 4 +p = 6720 +n = 3 +p = 20160 +n = 2 +p = 40320 +n = 1 +``` + +The procedural macro uses a syntax tree [`Fold`] to rewrite every `let` +statement and assignment expression in the following way: + +[`Fold`]: https://docs.rs/syn/1.0/syn/fold/trait.Fold.html + +```rust +// Before +let VAR = INIT; + +// After +let VAR = { let VAR = INIT; println!("VAR = {:?}", VAR); VAR }; +``` + +```rust +// Before +VAR = INIT + +// After +{ VAR = INIT; println!("VAR = {:?}", VAR); } +``` |