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An (incomplete) lisp for secure rule evaluation in Ruby
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A simple non-turing complete lisp meant to be embedded in apps as a rules engine. It is intentionally designed to limit the harm evaluated code can do (in contrast to a straight ruby 'eval') and is constrained to:

  • Be totally sandboxed by default
  • Always finish executing (no infinite loops)
  • Let you easily add variables and functions (simply pass in a hash defining them)
  • Be a small, single file


Either add the interrotron gem, or just copy and paste interrotron.rb


Interrotron can be executed either by calling the lib, or via the 'interrotron' executable that ships with the gem.

# Injecting a variable and evaluating a function is easy!'(> 51 custom_var)', :custom_var => 10) 
# => true

#You can inject functions just as easily"(doubler (+ 2 2))", :doubler => proc {|a| a*2 })
# => 8

# You can even pre-compile scripts for speed / re-use!
tron = => proc {|a| a.reverse == 'oof'})
compiled = tron.compile("(is_valid my_param)") => 'foo')
# => true => 'bar')
#=> false

# Since interrotron is meant for business rules, it handles dates as a 
# native type as instances of ruby's DateTime class. You can use literals
# for that like so:'(> #t{2010-09-04} start_date)', start_date: DateTime.parse('2012-12-12').to_time)
# => true'(> (now) (ago (hours 12)))')
# => true

# You can, of course, create arbitrarily complex exprs"(if false
                     (+ 4 -3)
                     (- 10 (+ 2 (+ 1 1))))")
# => 6

# Additionally, it is possible to constrain execution to a maximum number of
# operations by passing in a third argument"(str (+ 1 2) (+ 3 4) (+ 5 7))", {}, 3)
# => raises Interrotron::OpsThresholdError since 4 operations were executed

The following functions and variables are built in to Interrotron (and more are on the way!):

(if pred then else) ; it's an if / else statement
(cond pred1 clause1 pred2 clause2 true fallbackclause) ; like a case statement
(and e1, e2, ...) ; logical and, returns last arg if true
(or e1, e2, ...) ; logical or, returns first true arg
(not expr) ; negates
(lambda (arg1 arg2 ...) (e1) (e2)) ; creates an anonymous function that requires the given arguments and executes the expressions listed
(defn function_name (arg1 arg2 ...) (e1) (e2) ...) ; creates a named function in the global scope that requires the arguments and executes the expressions listed
(let (var_1 var_1_value var_2 var_2_value ...) (e1) (e2) ...) ; creates a new scope that sets the variables to their given values and executes the expressions listed
(setglobal var_name var_value) ; sets the variable name to the variable value in the global scope
(expr (e1) (e2) ...) ; executes the set of expressions in sequence
(! expr) ; negates
(identity expr) ; returns its argument
(str s1, s2, ...) ; converts its args to strings, also concatenates them
(floor expr) ; equiv to num.floor
(ceil expr) ; equiv to num.ceil
(round expr) ; equiv to num.round
(int expr) ; int conversion
(float expr) ; float conversion
(rand) ; returns a random float between 0 and 1
(upcase str) ; uppercases a string
(downcase) ; lowercases a string
(array e1, e2, ...) ; creates an array
(max arr) ; returns the largest element of an array
(min arr) ; returns the smallest element of an array
(length arr) ; get the length of an array
(first arr) ; get arr head
(last arr) ; get arr tail
(nth pos arr) ; get array at index
(member? val arr) ; check if the array has a member with value 'val'
(now) ; returns the current DateTime
(seconds n); n, for completeness
(minutes n); n * secs_in_a_minute
(hours n); n * secs_in_a_hour
(months n) ; n * secs_in_a_month
(ago n) ; yields a time in seconds from now
(from-now n) ; yields a time in seconds from now
(time 'str') ; parses a string to a Time


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request
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