From fff37e16acbc74c4e2891039415afed4c656d882 Mon Sep 17 00:00:00 2001
From: Nabeel Valley <36758308+nabeelvalley@users.noreply.github.com>
Date: Fri, 19 Jul 2024 22:13:12 +0200
Subject: [PATCH] :sparkles: maybe combinator

---
 src/parz/combinators.gleam  | 13 +++++++++++--
 test/combinators_test.gleam | 26 +++++++++++++++++++++-----
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/src/parz/combinators.gleam b/src/parz/combinators.gleam
index 849f168..36b9d6f 100644
--- a/src/parz/combinators.gleam
+++ b/src/parz/combinators.gleam
@@ -79,6 +79,15 @@ pub fn between(l: Parser, keep: Parser, r: Parser) -> Parser {
   }
 }
 
+pub fn maybe(parser) {
+  fn(input) {
+    case parser(input) {
+      Ok(ok) -> Ok(ok)
+      Error(_) -> Ok(ParserState("", input))
+    }
+  }
+}
+
 fn many_rec(parser: Parser, input, acc) {
   case parser(input) {
     Error(err) -> Error(err)
@@ -129,11 +138,11 @@ pub fn label_error(parser, message) {
   }
 }
 
-pub fn map(parser, transform) {
+pub fn map(parser: Parser, transform) {
   fn(input) {
     case parser(input) {
       Error(err) -> Error(err)
-      Ok(ok) -> Ok(transform(ok))
+      Ok(ok) -> Ok(#(transform(ok.matched), ok.remaining))
     }
   }
 }
diff --git a/test/combinators_test.gleam b/test/combinators_test.gleam
index 4185cd9..fd4d1fa 100644
--- a/test/combinators_test.gleam
+++ b/test/combinators_test.gleam
@@ -1,7 +1,8 @@
 import gleeunit/should
 import parz.{run}
 import parz/combinators.{
-  between, choice, concat, label_error, left, many, many1, map, right, sequence,
+  between, choice, concat, label_error, left, many, many1, map, maybe, right,
+  sequence,
 }
 import parz/parsers.{letters, str}
 import parz/types.{ParserState}
@@ -146,6 +147,22 @@ pub fn concat_test() {
   |> should.be_error
 }
 
+pub fn maybe_test() {
+  let parser = maybe(str("x"))
+
+  run(parser, "x")
+  |> should.be_ok
+  |> should.equal(ParserState("x", ""))
+
+  run(parser, "!")
+  |> should.be_ok
+  |> should.equal(ParserState("", "!"))
+
+  run(parser, "")
+  |> should.be_ok
+  |> should.equal(ParserState("", ""))
+}
+
 pub fn label_error_test() {
   let message = "Expected [letters]"
   let parser =
@@ -169,8 +186,7 @@ pub fn map_test() {
   let parser =
     concat(sequence([str("["), letters(), str("]")]))
     |> map(fn(ok) {
-      let seq = ok.matched
-      case seq {
+      case ok {
         "" -> NoContent
         content -> Content(content)
       }
@@ -178,11 +194,11 @@ pub fn map_test() {
 
   run(parser, "[hello]")
   |> should.be_ok
-  |> should.equal(Content("[hello]"))
+  |> should.equal(#(Content("[hello]"), ""))
 
   run(parser, "[hello]x")
   |> should.be_ok
-  |> should.equal(Content("[hello]"))
+  |> should.equal(#(Content("[hello]"), "x"))
 
   run(parser, "[hellox")
   |> should.be_error