util.interpolation: A template engine for text

util/interpolation.lua

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+-- Simple template language
+--
+-- The new() function takes a pattern and an escape function and returns 
+-- a render() function.  Both are required.
+--
+-- The function render() takes a string template and a table of values.
+-- Sequences like {name} in the template string are substituted
+-- with values from the table, optionally depending on a modifier
+-- symbol.
+--
+-- Variants are:
+-- {name} is substituted for values["name"] and is escaped using the 
+-- second argument to new_render().  To disable the escaping, use {name!}.
+-- {name.item} can be used to access table items.
+-- To renter lists of items: {name# item number {idx} is {item} }
+-- Or key-value pairs: {name% t[ {idx} ] = {item} }
+-- To show a defaults for missing values {name? sub-template } can be used, 
+-- which renders a sub-template if values["name"] is false-ish.
+-- {name& sub-template } does the opposite, the sub-template is rendered 
+-- if the selected value is anything but false or nil.
+
+local type, tostring = type, tostring;
+local pairs, ipairs = pairs, ipairs;
+local s_sub, s_gsub, s_match = string.sub, string.gsub, string.match;
+local t_concat = table.concat;
+
+local function new_render(pat, escape)
+	-- assert(type(pat) == "string", "bad argument #1 to 'new_render' (string expected)");
+	-- assert(type(escape) == "function", "bad argument #2 to 'new_render' (function expected)");
+	local function render(template, values)
+		-- assert(type(template) == "string", "bad argument #1 to 'render' (string expected)");
+		-- assert(type(values) == "table", "bad argument #2 to 'render' (table expected)");
+		return (s_gsub(template, pat, function (block)
+			block = s_sub(block, 2, -2);
+			local name, opt, e = s_match(block, "^([%a_][%w_.]*)(%p?)()");
+			if not name then return end
+			local value = values[name];
+			if not value and name:find(".", 2, true) then
+				value = values;
+				for word in name:gmatch"[^.]+" do
+					value = value[word];
+					if not value then break; end
+				end
+			end
+			if opt == '#' or opt == '%' then
+				if type(value) ~= "table" then return ""; end
+				local iter = opt == '#' and ipairs or pairs;
+				local out, i, subtpl = {}, 1, s_sub(block, e);
+				local subvalues = setmetatable({}, { __index = values });
+				for idx, item in iter(value) do
+					subvalues.idx = idx;
+					subvalues.item = item;
+					out[i], i = render(subtpl, subvalues), i+1;
+				end
+				return t_concat(out);
+			elseif opt == '&' then
+				if not value then return ""; end
+				return render(s_sub(block, e), values);
+			elseif opt == '?' and not value then
+				return render(s_sub(block, e), values);
+			elseif value ~= nil then
+				if type(value) ~= "string" then
+					value = tostring(value);
+				end
+				if opt ~= '!' then
+					return escape(value);
+				end
+				return value;
+			end
+		end));
+	end
+	return render;
+end
+
+return {
+	new = new_render;
+};