From a4a66a2e5be41832bcf23127a37515d155b700bc Mon Sep 17 00:00:00 2001
From: David Majda <david@majda.cz>
Date: Fri, 9 Oct 2015 17:21:33 +0200
Subject: [PATCH] Switch from first/rest to head/tail in the PEG.js grammar

In the past year I worked on various grammars where first/rest or
head/tail were used as labels for parts of lists. I found I associate
head/tail with a list immediately, while in case of first/rest I have to
"parse" grammar rules for a while before understanding their structure.

Moreover, I tend to assume that rest is a list of the same thigs as
first, but I don't have such assumption in case of head/tail. This
assumption was in conflict with the grammar structure.

I'm not sure how much these observations are applicable to others, but I
decided to act on them and switch from first/rest to head/tail.
---
 lib/parser.js    | 22 +++++++++++-----------
 src/parser.pegjs | 22 +++++++++++-----------
 2 files changed, 22 insertions(+), 22 deletions(-)

diff --git a/lib/parser.js b/lib/parser.js
index c8cdf5b..9e026b8 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -65,14 +65,14 @@ module.exports = (function() {
             },
         peg$c5 = "/",
         peg$c6 = { type: "literal", value: "/", description: "\"/\"" },
-        peg$c7 = function(first, rest) {
-              return rest.length > 0
+        peg$c7 = function(head, tail) {
+              return tail.length > 0
                 ? {
                     type:         "choice",
-                    alternatives: buildList(first, rest, 3),
+                    alternatives: buildList(head, tail, 3),
                     location:     location()
                   }
-                : first;
+                : head;
             },
         peg$c8 = function(expression, code) {
               return code !== null
@@ -84,14 +84,14 @@ module.exports = (function() {
                   }
                 : expression;
             },
-        peg$c9 = function(first, rest) {
-              return rest.length > 0
+        peg$c9 = function(head, tail) {
+              return tail.length > 0
                 ? {
                     type:     "sequence",
-                    elements: buildList(first, rest, 1),
+                    elements: buildList(head, tail, 1),
                     location: location()
                   }
-                : first;
+                : head;
             },
         peg$c10 = ":",
         peg$c11 = { type: "literal", value: ":", description: "\":\"" },
@@ -180,7 +180,7 @@ module.exports = (function() {
         peg$c67 = { type: "literal", value: "//", description: "\"//\"" },
         peg$c68 = function(name) { return name; },
         peg$c69 = { type: "other", description: "identifier" },
-        peg$c70 = function(first, rest) { return first + rest.join(""); },
+        peg$c70 = function(head, tail) { return head + tail.join(""); },
         peg$c71 = "_",
         peg$c72 = { type: "literal", value: "_", description: "\"_\"" },
         peg$c73 = "\\",
@@ -4920,8 +4920,8 @@ module.exports = (function() {
         return result;
       }
 
-      function buildList(first, rest, index) {
-        return [first].concat(extractList(rest, index));
+      function buildList(head, tail, index) {
+        return [head].concat(extractList(tail, index));
       }
 
 
diff --git a/src/parser.pegjs b/src/parser.pegjs
index 59edf87..13f33cf 100644
--- a/src/parser.pegjs
+++ b/src/parser.pegjs
@@ -67,8 +67,8 @@
     return result;
   }
 
-  function buildList(first, rest, index) {
-    return [first].concat(extractList(rest, index));
+  function buildList(head, tail, index) {
+    return [head].concat(extractList(tail, index));
   }
 }
 
@@ -114,14 +114,14 @@ Expression
   = ChoiceExpression
 
 ChoiceExpression
-  = first:ActionExpression rest:(__ "/" __ ActionExpression)* {
-      return rest.length > 0
+  = head:ActionExpression tail:(__ "/" __ ActionExpression)* {
+      return tail.length > 0
         ? {
             type:         "choice",
-            alternatives: buildList(first, rest, 3),
+            alternatives: buildList(head, tail, 3),
             location:     location()
           }
-        : first;
+        : head;
     }
 
 ActionExpression
@@ -137,14 +137,14 @@ ActionExpression
     }
 
 SequenceExpression
-  = first:LabeledExpression rest:(__ LabeledExpression)* {
-      return rest.length > 0
+  = head:LabeledExpression tail:(__ LabeledExpression)* {
+      return tail.length > 0
         ? {
             type:     "sequence",
-            elements: buildList(first, rest, 1),
+            elements: buildList(head, tail, 1),
             location: location()
           }
-        : first;
+        : head;
     }
 
 LabeledExpression
@@ -255,7 +255,7 @@ Identifier
   = !ReservedWord name:IdentifierName { return name; }
 
 IdentifierName "identifier"
-  = first:IdentifierStart rest:IdentifierPart* { return first + rest.join(""); }
+  = head:IdentifierStart tail:IdentifierPart* { return head + tail.join(""); }
 
 IdentifierStart
   = UnicodeLetter