From e16a4b989cc2275f7c8601bb44b6c21ff22e12e5 Mon Sep 17 00:00:00 2001
From: Undecidable Robot <undecidabot@gmail.com>
Date: Thu, 16 Jun 2016 00:51:22 +0800
Subject: Adding a simple example using 'expect'

---
 examples/expect.md | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 examples/expect.md

(limited to 'examples')

diff --git a/examples/expect.md b/examples/expect.md
new file mode 100644
index 0000000..544a55e
--- /dev/null
+++ b/examples/expect.md
@@ -0,0 +1,66 @@
+Here's an example of an LPegLabel grammar that make its own function called
+'expect', which takes a pattern and a label as parameters and throws the label
+if the pattern fails to be matched. This function can be extended later on to
+record all errors encountered once error recovery is implemented.
+
+```lua
+local lpeg = require"lpeglabel"
+
+local R, S, P, V, T = lpeg.R, lpeg.S, lpeg.P, lpeg.V, lpeg.T
+
+local labels = {
+  {"NoExp",     "no expression found"},
+  {"Extra",     "extra chracters found after the expression"},
+  {"ExpTerm",   "expected a term after the operator"},
+  {"ExpExp",    "expected an expression after the parenthesis"},
+  {"MisClose",  "missing a closing ')' after the expression"},
+}
+
+local function expect(patt, labname)
+  for i, elem in ipairs(labels) do
+    if elem[1] == labname then
+      return patt + T(i)
+    end
+  end
+
+  error("could not find label: " .. labname)
+end
+
+local num = R("09")^1
+local op = S("+-*/")
+
+local g = P {
+  "Exp",
+  Exp = V"Term" * (op * expect(V"Term", "ExpTerm"))^0;
+  Term = num + V"Group";
+  Group = "(" * expect(V"Exp", "ExpExp") * expect(")", "MisClose");
+}
+
+g = expect(g, "NoExp") * expect(-P(1), "Extra")
+
+local function check(input)
+  result, label, suffix = g:match(input)
+  if result ~= nil then
+    return "ok"
+  else
+    local pos = input:len() - suffix:len() + 1
+    local msg = labels[label][2]
+    return "syntax error: " .. msg .. " (at index " .. pos .. ")"
+  end
+end
+
+print(check "(1+1-1*2/2")
+--> syntax error: missing a closing ')' after the expression (at index 11)
+
+print(check "(1+)-1*(2/2)")
+--> syntax error: expected a term after the operator (at index 4)
+
+print(check "(1+1)-1*(/2)")
+--> syntax error: expected an expression after the parenthesis (at index 10)
+
+print(check "1+(1-(1*2))/2x")
+--> syntax error: extra chracters found after the expression (at index 14)
+
+print(check "-1+(1-(1*2))/2")
+--> syntax error: no expression found (at index 1)
+```
-- 
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