Our purpose in writing this tutorial is not to teach programming, nor even to teach functional programming. Rather, it is intended to serve as a supplement to the Haskell Report [4], which is otherwise a rather dense technical exposition. Our goal is to provide a gentle introduction to Haskell for someone who has experience with at least one other language, preferably a functional language (even if only an "almost-functional" language such as ML or Scheme). If the reader wishes to learn more about the functional programming style, we highly recommend Bird's text Introduction to Functional Programming [1] or Davie's An Introduction to Functional Programming Systems Using Haskell [2]. For a useful survey of functional programming languages and techniques, including some of the language design principles used in Haskell, see [3].
The Haskell language has evolved significantly since its birth in 1987. This tutorial deals with Haskell 98. Older versions of the language are now obsolete; Haskell users are encouraged to use Haskell 98. There are also many extensions to Haskell 98 that have been widely implemented. These are not yet a formal part of the Haskell language and are not covered in this tutorial.
Our general strategy for introducing language features is this: motivate the idea, define some terms, give some examples, and then point to the Report for details. We suggest, however, that the reader completely ignore the details until the Gentle Introduction has been completely read. On the other hand, Haskell's Standard Prelude (in Appendix A of the Report and the standard libraries (found in the Library Report [5]) contain lots of useful examples of Haskell code; we encourage a thorough reading once this tutorial is completed. This will not only give the reader a feel for what real Haskell code looks like, but will also familiarize her with Haskell's standard set of predefined functions and types.
Finally, the Haskell web site, http://haskell.org, has a wealth of information about the Haskell language and its implementations.
[We have also taken the course of not laying out a plethora of lexical syntax rules at the outset. Rather, we introduce them incrementally as our examples demand, and enclose them in brackets, as with this paragraph. This is in stark contrast to the organization of the Report, although the Report remains the authoritative source for details (references such as "report section 2.1" refer to sections in the Report).]
Haskell is a typeful programming language: (Coined by Luca Cardelli.) types are pervasive, and the newcomer is best off becoming well aware of the full power and complexity of Haskell's type system from the outset. For those whose only experience is with relatively "untypeful" languages such as Perl, Tcl, or Scheme, this may be a difficult adjustment; for those familiar with Java, C, Modula, or even ML, the adjustment should be easier but still not insignificant, since Haskell's type system is different and somewhat richer than most. In any case, "typeful programming" is part of the Haskell programming experience, and cannot be avoided.