Contracts describe properties of expressions (esp. functions) that are checked at run-time. For example, a contract states that an expression is a natural number, a list has at least one element or a function requires a natural number and a non-empty list of natural numbers as arguments and will produce a natural number as result. Thus these properties are both documented and enforced. Contracts are more expressive than static types. If a contract is violated, then an informative exception is raised. See homepage or Olaf Chitil: Practical Typed Lazy Contracts, ICFP 2012, ACM.

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