tower_layer/
stack.rs

1use super::Layer;
2use core::fmt;
3
4/// Two [`Layer`]s chained together.
5///
6/// # Examples
7///
8/// ```rust
9/// use tower_layer::{Stack, layer_fn, Layer};
10///
11/// let inner = layer_fn(|service| service+2);
12/// let outer = layer_fn(|service| service*2);
13///
14/// let inner_outer_stack = Stack::new(inner, outer);
15///
16/// // (4 + 2) * 2 = 12
17/// // (4 * 2) + 2 = 10
18/// assert_eq!(inner_outer_stack.layer(4), 12);
19/// ```
20#[derive(Clone)]
21pub struct Stack<Inner, Outer> {
22    inner: Inner,
23    outer: Outer,
24}
25
26impl<Inner, Outer> Stack<Inner, Outer> {
27    /// Creates a new [`Stack`].
28    ///
29    /// # Examples
30    ///
31    /// ```rust
32    /// use tower_layer::{Stack, Identity};
33    ///
34    /// let stack = Stack::new(Identity::new(), Identity::new());
35    /// ```
36    pub const fn new(inner: Inner, outer: Outer) -> Self {
37        Stack { inner, outer }
38    }
39}
40
41impl<S, Inner, Outer> Layer<S> for Stack<Inner, Outer>
42where
43    Inner: Layer<S>,
44    Outer: Layer<Inner::Service>,
45{
46    type Service = Outer::Service;
47
48    fn layer(&self, service: S) -> Self::Service {
49        let inner = self.inner.layer(service);
50
51        self.outer.layer(inner)
52    }
53}
54
55impl<Inner, Outer> fmt::Debug for Stack<Inner, Outer>
56where
57    Inner: fmt::Debug,
58    Outer: fmt::Debug,
59{
60    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
61        // The generated output of nested `Stack`s is very noisy and makes
62        // it harder to understand what is in a `ServiceBuilder`.
63        //
64        // Instead, this output is designed assuming that a `Stack` is
65        // usually quite nested, and inside a `ServiceBuilder`. Therefore,
66        // this skips using `f.debug_struct()`, since each one would force
67        // a new layer of indentation.
68        //
69        // - In compact mode, a nested stack ends up just looking like a flat
70        //   list of layers.
71        //
72        // - In pretty mode, while a newline is inserted between each layer,
73        //   the `DebugStruct` used in the `ServiceBuilder` will inject padding
74        //   to that each line is at the same indentation level.
75        //
76        // Also, the order of [outer, inner] is important, since it reflects
77        // the order that the layers were added to the stack.
78        if f.alternate() {
79            // pretty
80            write!(f, "{:#?},\n{:#?}", self.outer, self.inner)
81        } else {
82            write!(f, "{:?}, {:?}", self.outer, self.inner)
83        }
84    }
85}
tower_layer/stack.rs

tower_layer/
stack.rs
