@TECHREPORT{Wadler87theconcatenate, author = {Philip Wadler}, title = {The Concatenate Vanishes}, institution = {}, year = {1987} }

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Abstract

This note presents a trivial transformation that can eliminate many calls of the concatenate (or “append”) operator from a program. The general form of the transformation is well known, and one of the examples, transforming the reverse function, is a classic. However, so far as I am aware, this style of transformation has not previously been systematised in the way done here. The transformation is suitable for incorporation in a compiler, and improves the asymptotic time complexity of some programs from quadratic to linear. There is a syntactic test that determines when the transformation will succeed in eliminating a concatenate operation. Section 1 describes the transformation. Section 2 presents examples. Section 3 characterises the transformation’s benefits. Section 4 describes related work. Section 5 concludes. 1 The transformation First, some notational preliminaries. We write concatenate as infix +, list construction (cons) as infix:, and the empty list as []. We write [x, y, z] as an abbreviation for x: (y: (z: [])). We will make use of the following laws: (1) [ ] + x = x (2) (x: y) + z = x: (y + z) (3) (x + y) + z = x + (y + z) Laws (1) and (2) provide a recursive definition of concatenate. Law (3) states that concatenate is associative; it may be proved from laws (1) and (2). We now describe the transformation. The key idea is that whenever an application of a function f may appear as the left argument of a concatenation, then we introduce a new function f ′ , satisfying the property