MLFis a type system that seamlessly merges ML-style implicit but second-class polymorphism with System-F explicit first-class polymorphism. We present xMLF, a Church-style version of MLF with full type information that can easily be maintained during reduction. All parameters of functions are explicitly typed and both type abstraction and type instantiation are explicit. However, type instantiation in xMLF is more general than type application in System F. We equip xMLF with a small-step reduction semantics that allows reduction in any context, and show that this relation is confluent and type preserving. We also show that both subject reduction and progress hold for weak-reduction strategies, including call-by-value with the value-restriction. We exhibit a type preserving encoding of MLF into xMLF, which shows that xMLF can be used as the internal language for MLF after type inference, and also ensures type soundness for the most expressive variant of MLF.

Static type systems strive to be richly expressive while still being simple enough for programmers to use. We describe an experiment that enriches Haskell’s kind system with two features promoted from its type system: data types and polymorphism. The new system has a very good power-to-weight ratio: it offers a significant improvement in expressiveness, but, by re-using concepts that programmers are already familiar with, the system is easy to understand and implement. Categories and Subject Descriptors D.3.3 [Language Constructs and Features]: Data types and structures, Polymorphism; F.3.3 [Studies of Program Constructs]: Type structure

proofs and deferred type errors

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Though Haskell is predominantly type-safe, implementations contain a few loopholes through which code can bypass typing and module encapsulation. This paper presents Safe Haskell, a language extension that closes these loopholes. Safe Haskell makes it possible to confine and safely execute untrusted, possibly malicious code. By strictly enforcing types, Safe Haskell allows a variety of different policies from API sandboxing to information-flow control to be implemented easily as monads. Safe Haskell is aimed to be as unobtrusive as possible. It enforces properties that programmers tend to meet already by convention. We describe the design of Safe Haskell and an implementation (currently shipping with GHC) that infers safety for code that lies in a safe subset of the language. We use Safe Haskell to implement an online Haskell interpreter that can securely execute arbitrary untrusted code with no overhead. The use of Safe Haskell greatly simplifies this task and allows the use of a large body of existing code and tools.

Though Haskell is predominantly type-safe, implementations contain a few loopholes through which code can bypass typing and module encapsulation. This paper presents Safe Haskell, a language extension that closes these loopholes. Safe Haskell makes it possible to confine and safely execute untrusted, possibly malicious code. By strictly enforcing types, Safe Haskell allows a variety of different policies from API sandboxing to information-flow control to be implemented easily as monads. Safe Haskell is aimed to be as unobtrusive as possible. It enforces properties that programmers tend to meet already by convention. We describe the design of Safe Haskell and an implementation (currently shipping with GHC) that infers safety for code that lies in a safe subset of the language. We use Safe Haskell to implement an online Haskell interpreter that can securely execute arbitrary untrusted code with no overhead. The use of Safe Haskell greatly simplifies this task and allows the use of a large body of existing code and tools.