From e5dc04764c2c48fdd98c5a68b4d602f6bdd914e3 Mon Sep 17 00:00:00 2001
From: Phil Hagelberg <phil@hagelb.org>
Date: Sun, 5 Jan 2025 12:40:25 -0800
Subject: [PATCH] Allow trailing underscores to bypass unused local warnings.

---
 docs/manual.md         | 15 ++++++++++++++-
 src/fennel-ls/lint.fnl |  7 +------
 test/lint.fnl          |  4 +++-
 3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/docs/manual.md b/docs/manual.md
index 759fc03..0870418 100644
--- a/docs/manual.md
+++ b/docs/manual.md
@@ -13,6 +13,9 @@ $ make
 ```
 will create a bleeding-edge latest git `fennel-ls` binary for you.
 
+Run `make install PREFIX=$HOME` to put it in `~/bin` or `sudo make install` for
+a systemwide install.
+
 #### Arch Linux
 I think `fennel-ls` and `fennel-ls-git` may be in the AUR.
 
@@ -128,10 +131,20 @@ Goto Reference  | [X]    | [ ]    | [ ]             | [ ]          | [ ]      |
 
 Fennel-ls can report all fennel compiler errors, plus a few custom lints.
 
+Unused locals will be flagged unless they begin or end with an underscore. If
+you have a local that is unused in your code but necessary for pattern matching
+purposes, it's recommended to put an underscore at the end. For example:
+
+```fennel
+(case [1 1 2 3 5 8]
+  [a_ a_] (print "First two elements are equal"))
+```
+
 ## CLI Usage
 ```sh
 fennel-ls --lint my-file.fnl f2.fnl # prints diagnostics for the files given
 ```
 
-This will analyze the given files, and print out all compiler errors and lints, without launching a server.
+This will analyze the given files, and print out all compiler errors and lints,
+without launching a server. A successful exit code indicates no problems found.
 
diff --git a/src/fennel-ls/lint.fnl b/src/fennel-ls/lint.fnl
index 0efde9a..e4167da 100644
--- a/src/fennel-ls/lint.fnl
+++ b/src/fennel-ls/lint.fnl
@@ -32,6 +32,7 @@ the `file.diagnostics` field, filling it with diagnostics."
 (λ unused-definition [server file symbol definition]
   "local variable that is defined but not used"
   (if (not (or (= "_" (: (tostring symbol) :sub 1 1))
+               (= "_" (: (tostring symbol) :sub -1 -1))
                (accumulate [reference false
                             _ ref (ipairs definition.referenced-by)
                             &until reference]
@@ -249,11 +250,5 @@ the `file.diagnostics` field, filling it with diagnostics."
 
     (if lints.unknown-module-field
       (unknown-module-field server file))))
-    ;; (if lints.unnecessary-values
-    ;;   (unnecessary-values file)))
-    ;; (if lints.unnecessary-do)
-    ;;   (unnecessary-do file)))
-    ;; (if lints.unnecessary-unary-op))
-    ;;   (unnecessary-values file)))
 
 {: add-lint-diagnostics}
diff --git a/test/lint.fnl b/test/lint.fnl
index 50aebdf..49058a9 100644
--- a/test/lint.fnl
+++ b/test/lint.fnl
@@ -55,6 +55,8 @@
          [{:code 301
            :range {:start {:character 9  :line 0}
                    :end   {:character 10 :line 0}}}])
+  (check "(case [1 1 2 3 5 8] [a a] (print :first-two-equal))" [{:code 301}])
+  (assert-ok "(case [1 1 2 3 5 8] [a_ a_] (print :first-two-equal))")
   ;; setting a var without reading
   (check "(var x 1) (set x 2) (set [x] [3])"
           [{:code 301
@@ -248,7 +250,7 @@
 ;; duplicate keys in kv table
 ;; (tset <sym> <any>) --> (set (. <sym> <any>)) (might be wanted for compat?)
 ;; {&as x} and [&as x] pattern with no other matches
-;; Unused variables / fields (maybe difficult)
+;; Unused fields (maybe difficult)
 ;; discarding results to various calls, such as unpack, values, etc
 ;; `pairs` or `ipairs` call in a (for) binding table
 ;; steal as many lints as possible from cargo