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A new kind of attribute grammars, called higher order attribute grammars, is defined. In higher order attribute grammars the structure tree can be expanded as a result of attribute computation. A structure tree may be stored in an attribute. The term higher order is used because of the analogy with higher order functions, where a function can be the result or parameter of another function. A relatively simple method, using OAGs, is described to derive an evaluation order on the defining attribute occurrences which comprises all possible direct and indirect attribute dependencies. As in OAGs, visit-sequences are computed from which an efficient algorithm for attribute evaluation can be derived.

In this paper we describe the architecture of the Utrecht Haskell Compiler (UHC). UHC is a new Haskell compiler, that supports most (but not all) Haskell 98 features, plus some experimental extensions. It targets multiple backends, including a bytecode interpreter backend and a whole-program analysis backend, both via C. The implementation is rigorously organized as stepwise transformations through some explicit intermediate languages. The tree walks of all transformations are expressed as an algebra, with the aid of an Attribute Grammar based preprocessor. The compiler is just one materialization of a framework that supports experimentation with language variants, thanks to an aspect-oriented internal organization.