From 4ebafb900561babe780e3f3ee06597acff33f885 Mon Sep 17 00:00:00 2001 From: Jonas Schievink Date: Sun, 14 Jun 2020 15:56:02 +0200 Subject: _match.rs: improve comment formatting --- crates/ra_hir_ty/src/_match.rs | 255 +++++++++++++++++++++++------------------ 1 file changed, 142 insertions(+), 113 deletions(-) (limited to 'crates/ra_hir_ty') diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs index 3e6e1e333..fff257193 100644 --- a/crates/ra_hir_ty/src/_match.rs +++ b/crates/ra_hir_ty/src/_match.rs @@ -8,11 +8,11 @@ //! This file includes the logic for exhaustiveness and usefulness checking for //! pattern-matching. Specifically, given a list of patterns for a type, we can //! tell whether: -//! (a) the patterns cover every possible constructor for the type [exhaustiveness] -//! (b) each pattern is necessary [usefulness] +//! - (a) the patterns cover every possible constructor for the type (exhaustiveness). +//! - (b) each pattern is necessary (usefulness). //! -//! The algorithm implemented here is a modified version of the one described in: -//! http://moscova.inria.fr/~maranget/papers/warn/index.html +//! The algorithm implemented here is a modified version of the one described in +//! . //! However, to save future implementors from reading the original paper, we //! summarise the algorithm here to hopefully save time and be a little clearer //! (without being so rigorous). @@ -37,20 +37,26 @@ //! new pattern `p`. //! //! For example, say we have the following: +//! +//! ```ignore +//! // x: (Option, Result<()>) +//! match x { +//! (Some(true), _) => {} +//! (None, Err(())) => {} +//! (None, Err(_)) => {} +//! } //! ``` -//! // x: (Option, Result<()>) -//! match x { -//! (Some(true), _) => {} -//! (None, Err(())) => {} -//! (None, Err(_)) => {} -//! } -//! ``` +//! //! Here, the matrix `P` starts as: +//! +//! ```text //! [ //! [(Some(true), _)], //! [(None, Err(()))], //! [(None, Err(_))], //! ] +//! ``` +//! //! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering //! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because //! all the values it covers are already covered by row 2. @@ -60,53 +66,61 @@ //! To match the paper, the top of the stack is at the beginning / on the left. //! //! There are two important operations on pattern-stacks necessary to understand the algorithm: -//! 1. We can pop a given constructor off the top of a stack. This operation is called -//! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or -//! `None`) and `p` a pattern-stack. -//! If the pattern on top of the stack can cover `c`, this removes the constructor and -//! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns. -//! Otherwise the pattern-stack is discarded. -//! This essentially filters those pattern-stacks whose top covers the constructor `c` and -//! discards the others. //! -//! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we -//! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the -//! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get -//! nothing back. +//! 1. We can pop a given constructor off the top of a stack. This operation is called +//! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or +//! `None`) and `p` a pattern-stack. +//! If the pattern on top of the stack can cover `c`, this removes the constructor and +//! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns. +//! Otherwise the pattern-stack is discarded. +//! This essentially filters those pattern-stacks whose top covers the constructor `c` and +//! discards the others. +//! +//! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we +//! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the +//! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get +//! nothing back. +//! +//! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1` +//! on top of the stack, and we have four cases: +//! +//! * 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We push onto +//! the stack the arguments of this constructor, and return the result: +//! +//! r_1, .., r_a, p_2, .., p_n +//! +//! * 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and return +//! nothing. +//! * 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has +//! arguments (its arity), and return the resulting stack: //! -//! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1` -//! on top of the stack, and we have four cases: -//! 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We -//! push onto the stack the arguments of this constructor, and return the result: -//! r_1, .., r_a, p_2, .., p_n -//! 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and -//! return nothing. -//! 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has -//! arguments (its arity), and return the resulting stack: -//! _, .., _, p_2, .., p_n -//! 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting -//! stack: -//! S(c, (r_1, p_2, .., p_n)) -//! S(c, (r_2, p_2, .., p_n)) +//! _, .., _, p_2, .., p_n //! -//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is -//! a pattern-stack. -//! This is used when we know there are missing constructor cases, but there might be -//! existing wildcard patterns, so to check the usefulness of the matrix, we have to check -//! all its *other* components. +//! * 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack: //! -//! It is computed as follows. We look at the pattern `p_1` on top of the stack, -//! and we have three cases: -//! 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing. -//! 1.2. `p_1 = _`. We return the rest of the stack: -//! p_2, .., p_n -//! 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting -//! stack. -//! D((r_1, p_2, .., p_n)) -//! D((r_2, p_2, .., p_n)) +//! S(c, (r_1, p_2, .., p_n)) +//! S(c, (r_2, p_2, .., p_n)) //! -//! Note that the OR-patterns are not always used directly in Rust, but are used to derive the -//! exhaustive integer matching rules, so they're written here for posterity. +//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is +//! a pattern-stack. +//! This is used when we know there are missing constructor cases, but there might be +//! existing wildcard patterns, so to check the usefulness of the matrix, we have to check +//! all its *other* components. +//! +//! It is computed as follows. We look at the pattern `p_1` on top of the stack, +//! and we have three cases: +//! * 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing. +//! * 1.2. `p_1 = _`. We return the rest of the stack: +//! +//! p_2, .., p_n +//! +//! * 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack: +//! +//! D((r_1, p_2, .., p_n)) +//! D((r_2, p_2, .., p_n)) +//! +//! Note that the OR-patterns are not always used directly in Rust, but are used to derive the +//! exhaustive integer matching rules, so they're written here for posterity. //! //! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by //! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with @@ -120,73 +134,88 @@ //! operates principally on the first component of the matrix and new pattern-stack `p`. //! This algorithm is realised in the `is_useful` function. //! -//! Base case. (`n = 0`, i.e., an empty tuple pattern) -//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), -//! then `U(P, p)` is false. -//! - Otherwise, `P` must be empty, so `U(P, p)` is true. +//! Base case (`n = 0`, i.e., an empty tuple pattern): +//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), then +//! `U(P, p)` is false. +//! - Otherwise, `P` must be empty, so `U(P, p)` is true. +//! +//! Inductive step (`n > 0`, i.e., whether there's at least one column [which may then be expanded +//! into further columns later]). We're going to match on the top of the new pattern-stack, `p_1`: +//! +//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern. +//! Then, the usefulness of `p_1` can be reduced to whether it is useful when +//! we ignore all the patterns in the first column of `P` that involve other constructors. +//! This is where `S(c, P)` comes in: +//! +//! ```text +//! U(P, p) := U(S(c, P), S(c, p)) +//! ``` +//! +//! This special case is handled in `is_useful_specialized`. +//! +//! For example, if `P` is: +//! +//! ```text +//! [ +//! [Some(true), _], +//! [None, 0], +//! ] +//! ``` //! -//! Inductive step. (`n > 0`, i.e., whether there's at least one column -//! [which may then be expanded into further columns later]) -//! We're going to match on the top of the new pattern-stack, `p_1`. -//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern. -//! Then, the usefulness of `p_1` can be reduced to whether it is useful when -//! we ignore all the patterns in the first column of `P` that involve other constructors. -//! This is where `S(c, P)` comes in: -//! `U(P, p) := U(S(c, P), S(c, p))` -//! This special case is handled in `is_useful_specialized`. +//! and `p` is `[Some(false), 0]`, then we don't care about row 2 since we know `p` only +//! matches values that row 2 doesn't. For row 1 however, we need to dig into the +//! arguments of `Some` to know whether some new value is covered. So we compute +//! `U([[true, _]], [false, 0])`. //! -//! For example, if `P` is: -//! [ -//! [Some(true), _], -//! [None, 0], -//! ] -//! and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only -//! matches values that row 2 doesn't. For row 1 however, we need to dig into the -//! arguments of `Some` to know whether some new value is covered. So we compute -//! `U([[true, _]], [false, 0])`. +//! - If `p_1 == _`, then we look at the list of constructors that appear in the first component of +//! the rows of `P`: +//! - If there are some constructors that aren't present, then we might think that the +//! wildcard `_` is useful, since it covers those constructors that weren't covered +//! before. +//! That's almost correct, but only works if there were no wildcards in those first +//! components. So we need to check that `p` is useful with respect to the rows that +//! start with a wildcard, if there are any. This is where `D` comes in: +//! `U(P, p) := U(D(P), D(p))` //! -//! - If `p_1 == _`, then we look at the list of constructors that appear in the first -//! component of the rows of `P`: -//! + If there are some constructors that aren't present, then we might think that the -//! wildcard `_` is useful, since it covers those constructors that weren't covered -//! before. -//! That's almost correct, but only works if there were no wildcards in those first -//! components. So we need to check that `p` is useful with respect to the rows that -//! start with a wildcard, if there are any. This is where `D` comes in: -//! `U(P, p) := U(D(P), D(p))` +//! For example, if `P` is: +//! ```text +//! [ +//! [_, true, _], +//! [None, false, 1], +//! ] +//! ``` +//! and `p` is `[_, false, _]`, the `Some` constructor doesn't appear in `P`. So if we +//! only had row 2, we'd know that `p` is useful. However row 1 starts with a +//! wildcard, so we need to check whether `U([[true, _]], [false, 1])`. //! -//! For example, if `P` is: -//! [ -//! [_, true, _], -//! [None, false, 1], -//! ] -//! and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we -//! only had row 2, we'd know that `p` is useful. However row 1 starts with a -//! wildcard, so we need to check whether `U([[true, _]], [false, 1])`. +//! - Otherwise, all possible constructors (for the relevant type) are present. In this +//! case we must check whether the wildcard pattern covers any unmatched value. For +//! that, we can think of the `_` pattern as a big OR-pattern that covers all +//! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for +//! example. The wildcard pattern is useful in this case if it is useful when +//! specialized to one of the possible constructors. So we compute: +//! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))` //! -//! + Otherwise, all possible constructors (for the relevant type) are present. In this -//! case we must check whether the wildcard pattern covers any unmatched value. For -//! that, we can think of the `_` pattern as a big OR-pattern that covers all -//! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for -//! example. The wildcard pattern is useful in this case if it is useful when -//! specialized to one of the possible constructors. So we compute: -//! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))` +//! For example, if `P` is: +//! ```text +//! [ +//! [Some(true), _], +//! [None, false], +//! ] +//! ``` +//! and `p` is `[_, false]`, both `None` and `Some` constructors appear in the first +//! components of `P`. We will therefore try popping both constructors in turn: we +//! compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]], +//! [false])` for the `None` constructor. The first case returns true, so we know that +//! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched +//! before. //! -//! For example, if `P` is: -//! [ -//! [Some(true), _], -//! [None, false], -//! ] -//! and `p` is [_, false], both `None` and `Some` constructors appear in the first -//! components of `P`. We will therefore try popping both constructors in turn: we -//! compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]], -//! [false]) for the `None` constructor. The first case returns true, so we know that -//! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched -//! before. +//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately: //! -//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately: -//! `U(P, p) := U(P, (r_1, p_2, .., p_n)) -//! || U(P, (r_2, p_2, .., p_n))` +//! ```text +//! U(P, p) := U(P, (r_1, p_2, .., p_n)) +//! || U(P, (r_2, p_2, .., p_n)) +//! ``` use std::sync::Arc; use smallvec::{smallvec, SmallVec}; -- cgit v1.2.3