From f156d80cc3c80385456bc3ec80adb5ad892f5039 Mon Sep 17 00:00:00 2001
From: Matthew Hall <hallmatthew314@gmail.com>
Date: Sun, 26 Mar 2023 15:37:26 +1300
Subject: Implement map_const

---
 spec/parcom_spec.cr  | 12 ++++++++++++
 src/parcom/parser.cr | 12 ++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index efb3964..cee6ff4 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -278,6 +278,18 @@ describe Parser do
     end
   end
 
+  describe "#map_const" do
+    p = Parser(Char, Char).any_token
+    p_mapped = p.map_const(42)
+
+    tokens = Tokens.from_string("abcd")
+    result = p_mapped.parse(tokens)
+
+    it "changes the value of the result" do
+      result.value.should eq(42)
+    end
+  end
+
   describe "#|" do
     a = Parser(Char, Char).token('a')
     b = Parser(Char, Char).token('b')
diff --git a/src/parcom/parser.cr b/src/parcom/parser.cr
index fd161f9..57c7b44 100644
--- a/src/parcom/parser.cr
+++ b/src/parcom/parser.cr
@@ -207,6 +207,18 @@ module Parcom
       map(block)
     end
 
+    # Replaces the value inside `self` when parsing is successful.
+    # This is effectively a shorthand for the following:
+    # ```
+    # a = Parser.token('a')
+    # # These are equivalent:
+    # into42_map       = a.map { |_| 42 }
+    # into42_map_const = a.map_const(42)
+    # ```
+    def map_const(value : V) : Parser(T, V) forall V
+      map { |_| value }.named("#{@name} (const #{value})")
+    end
+
     # Creates a new parser from `self` and another parser that will
     # try to parse with either of them. If the first parser succeeds,
     # it will return the result of the first parser. Otherwise, it will
-- 
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