Ellis Kesterton & Matthew Pickering - Intensional Analysis of Typed Template Haskell Quotations

Typed Template Haskell allows us to write Haskell code which generates other Haskell programs in a type-safe and principled manner. However, the generated programs are completely opaque and cannot be introspected, limiting the type of analysis and transformations that we can perform. We propose a system which allows the programmer to overload the meaning of quoted Template Haskell expressions by desugaring these expressions into a well-typed PHOAS representation. Being a regular datatype, the PHOAS representation is much more amenable to analysis and transformation, indirectly giving the programmer the ability to overload the meaning of quoted expressions by further processing the PHOAS representation. Primitive Haskell constructs such as variables (both free and bound), lambda expressions and patterns are all exposed in the PHOAS interface, giving the programmer a large amount of control over the meaning of their quoted expressions. We believe that this system has a variety of useful applications, particularly for creating EDSLs - we give motivating examples in practical areas such as distributed computing and program generation to demonstrate the effectiveness of this approach. More precisely, in this paper we discuss the design and implementation of this idea as a new Haskell extension implemented on top of GHC.