diff --git a/faces.lua b/faces.lua
index 11c7a95..f321c1a 100644
--- a/faces.lua
+++ b/faces.lua
@@ -31,12 +31,48 @@ local faces = {}
 -- STATIC FUNCTIONS --
 ----------------------
 
+-- inspired by Penlight string_lambda:
+-- http://stevedonovan.github.io/Penlight/api/libraries/pl.utils.html#string_lambda
+local lambda
+do
+  local memory = {}
+  lambda = setmetatable({ clear = function() memory = {} end, },
+    {
+      __call = function(self,fstr)
+        local fun = memory[fstr]
+        if not fun then
+          local args,code = fstr:match("^%s*|(.+)|(.+)$")
+          assert(args and code, "Needs args and code sections, e.g., |args|code")
+          local fun_src = ("return function(%s) return %s end"):format(args,code)
+          fun = assert(loadstring(fun_src))()
+          memory[fstr] = fun
+        end
+        return fun      
+      end,
+                       })
+end
+
+-- transform a user function string into a lambda function
+local function lambda_transform(func_str)
+  return function(vars)
+    local values,keys = {},{}
+    for k,v in pairs(vars) do
+      values[#values+1] = v
+      keys[#keys+1] = k
+    end
+    local code = ('|%s|%s'):format(table.concat(keys,","),func_str)
+    local f = lambda(code)
+    return f(table.unpack(values))
+  end
+end
+
 -- converts to in-mutable the given table argument
 local function inmutable(tbl)
   return setmetatable({}, {
       __index = function(_,k) return tbl[k] end,
       __newindex = function() error("Unable to modify an in-mutable table") end,
       __len = function() return #tbl end,
+      __pairs = function() return pairs(tbl) end,
   })
 end
 
@@ -131,15 +167,16 @@ do
   end
 
   assign_variables = function(self, vars, patterns, sequence, var_matches,
-                              user_clauses)
+                              user_clauses, fact_vars)
     for i,pat in ipairs(patterns) do
       local fid  = sequence[i]
       local fact = self.fact_list[fid]
       if not assign_fact_vars(vars, pat, fact, var_matches) then return false end
     end
+    for vname,i in pairs(fact_vars) do vars[vname] = sequence[i] end
     local inmutable_vars = inmutable(vars)
     for _,func in ipairs(user_clauses) do
-      if not func(sequence, inmutable_vars) then return false end
+      if not func(inmutable_vars) then return false end
     end
     return true
   end
@@ -186,7 +223,8 @@ local function regenerate_agenda(self)
         if not rule_entailements[sequence] then
           local seq_vars = {}
           if assign_variables(self, seq_vars, rule.patterns, sequence,
-                              rule.var_matches, rule.user_clauses) then
+                              rule.var_matches, rule.user_clauses,
+                              rule.fact_vars) then
             table.insert(combinations, sequence)
             table.insert(variables, seq_vars)
           end
@@ -231,7 +269,7 @@ local function fire_rule(self, rule_name, args, vars)
   for i,v in ipairs(args) do self.fact_entailment[v] = args end
   -- execute rule actions
   for _,action in ipairs(rule.actions) do
-    action(args, inmutable(vars))
+    action(inmutable(vars))
   end
 end
 
@@ -464,14 +502,27 @@ end
 -- declares a new rule in the knowledge base
 function faces_methods:defrule(rule_name)
   local rule = { patterns={}, user_clauses = {},
-                 actions={}, salience=0, var_matches = {} }
+                 actions={}, salience=0, var_matches = {}, fact_vars = {} }
   self.kb_table[rule_name] = rule
-  local rule_builder = {
+  local rule_builder
+  rule_builder = {
     pattern = function(rule_builder, pattern)
       table.insert(rule.patterns, tuple(pattern))
       return rule_builder
     end,
+    var = function(rule_builder, varname)
+      varname = assert(varname:match("%?([^%s]+)"),
+                       string.format("Incorrect variable name: %s", varname))
+      rule.fact_vars[varname] = #rule.patterns + 1
+      return {
+        pattern = function(_,...)
+          return rule_builder.pattern(rule_builder,...)
+        end
+      }
+    end,
     u = function(rule_builder, func)
+      -- user_func receives one argument: vars
+      if type(func) == "string" then func = lambda_transform(func) end
       table.insert(rule.user_clauses, func)
       return rule_builder
     end,
@@ -502,7 +553,8 @@ function faces_methods:defrule(rule_name)
           __index = function(rule_builder, key)
             if key == "u" then
               return function(rule_builder, user_func)
-                -- user_func receives two arguments (fact_ids, vars)
+                -- user_func receives one argument: vars
+                if type(func) == "string" then func = lambda_transform(func) end
                 table.insert(rule.actions, user_func)
                 return rule_builder
               end
@@ -514,7 +566,7 @@ function faces_methods:defrule(rule_name)
                 local args = table.pack(...)
                 for i=1,args.n do args[i] = tuple(args[i]) end
                 table.insert(rule.actions,
-                             function(fact_ids, vars)
+                             function(vars)
                                local new_args = replace_variables(args, vars)
                                return self[key](self, table.unpack(new_args))
                 end)
diff --git a/tests/animals.lua b/tests/animals.lua
index 1c446ef..d824af2 100644
--- a/tests/animals.lua
+++ b/tests/animals.lua
@@ -78,11 +78,11 @@ kb:defrule("Error"):
 -- Retracts error rule in case any classification is asserted
 kb:defrule("RetractError"):
   salience(100):
-  pattern{ "ERROR" }:
+  var("?f1"):pattern{ "ERROR" }:
   pattern{ ANIMAL_IS, ".*" }:
   ENTAILS("=>"):
-  u(function(fact_ids, vars)
-      kb:retract(fact_ids[1])
+  u(function(vars)
+      kb:retract(vars.f1)
   end)
 
 -- Initial rule, asks the first question
@@ -149,7 +149,7 @@ kb:defrule("Show"):
   salience(-10):
   pattern{ ANIMAL_IS, "?x" }:
   ENTAILS("=>"):
-  u(function(fact_ids,vars)
+  u(function(vars)
       print("The animal is a: " .. vars.x)
   end)
 
diff --git a/tests/factorial.lua b/tests/factorial.lua
index 8e20c6e..5cb85ad 100644
--- a/tests/factorial.lua
+++ b/tests/factorial.lua
@@ -4,11 +4,11 @@ local kb = faces()
 kb:defrule("expand"):
   salience(100):
   pattern{ "Factorial", "?x" }:
-  u(function(fact_ids, vars)
+  u(function(vars)
       return vars.x > 1
   end):
   ENTAILS("=>"):
-  u(function(fact_ids, vars)
+  u(function(vars)
       kb:fassert{ "Factorial", vars.x-1 }
   end)
 
@@ -16,11 +16,11 @@ kb:defrule("compute"):
   salience(0):
   pattern{ "Factorial", "?x" }:
   pattern{ "Result", "?y", "?z" }:
-  u(function(fact_ids, vars)
+  u(function(vars)
       return vars.x == vars.y+1
   end):
   ENTAILS("=>"):
-  u(function(fact_ids, vars)
+  u(function(vars)
       kb:fassert{ "Result", vars.x, vars.x * vars.z }
   end)
 
diff --git a/tests/test.lua b/tests/test.lua
index 45f234d..9a42e3f 100644
--- a/tests/test.lua
+++ b/tests/test.lua
@@ -58,12 +58,12 @@ kb:fassert{ "duck sound", "poww" }
 
 kb:defrule("duck2"):
   salience(100):
-  pattern{ "duck sound", { "?name1", "?name2" } }:
+  var("?f1"):pattern{ "duck sound", { "?name1", "?name2" } }:
   match("?name1", "q.*"):
   ENTAILS("=>"):
   fassert{ "sound is", { "?name1", "?name2" } }:
-  u(function(fact_ids, vars)
-      print("DEBUG", kb:consult(fact_ids[1]), vars.name1, vars.name2)
+  u(function(vars)
+      print("DEBUG", kb:consult(vars.f1), vars.name1, vars.name2)
   end)
 
 kb:defrule("init"):
@@ -99,7 +99,7 @@ kb:facts()
 kb:defrule("MultiValuated"):
   pattern{ "duck sound", "$?p" }:
   ENTAILS("=>"):
-  u(function(fact_ids, vars)
+  u(function(vars)
       print(vars.p)
   end)
 kb:agenda()
diff --git a/tests/test2.lua b/tests/test2.lua
index 7f9f66f..6db23f1 100644
--- a/tests/test2.lua
+++ b/tests/test2.lua
@@ -12,9 +12,7 @@ kb:defrule("user"):
   pattern{ "AnimalIs", "?x" }:
   numeric("?y"):
   numeric("?z"):
-  u(function(fact_ids, vars)
-      return (vars.z == vars.y*4) and (vars.y % 2)==0
-  end):
+  u("(z == y*4) and (y%2)==0"):
   ENTAILS("=>"):
   fassert{ "EvenAnimal", "?x", "?y", "?z" }