The Incomplete Guide to Lazy Evaluation in Haskell (2015)

# The Incomplete Guide to Lazy Evaluation (in Haskell) Source: [https://apfelmus.nfshost.com/articles/lazy-eval.html](https://apfelmus.nfshost.com/articles/lazy-eval.html) *Originally, I wanted to write a complete guide to lazy evaluation, but then\.* Lazy evaluation is the most widely used method for executing Haskell program code on a computer\. It determines the time and memory usage of Haskell programs, and it allows new and powerful ways to write modular code\. To make full use of purely functional programming, a good understanding of lazy evaluation is very helpful\. The following series of tutorials provide a thorough introduction to this topic: - [How does Lazy Evaluation Work in Haskell?](https://apfelmus.nfshost.com/articles/lazy-eval-intro.html)— Start here\. This tutorial explains how expressions are evaluated using lazy evaluation, and what that means for time and memory usage\. - [Writing More Modular Code with Lazy Evaluation](https://apfelmus.nfshost.com/articles/lazy-eval-modular-code.html)— Why use lazy evaluation in the first place? This tutorial explains the power of lazy evaluation and how it can help us to write clear and modular code\. In particular, it will tell you about**infinite lists**\. - [Non\-Strict Semantics of Haskell – What Exactly does Lazy Evaluation Calculate?](https://apfelmus.nfshost.com/articles/non-strict-semantics.html)– This tutorial explains that we actually do*not*need to know how lazy evaluation works in order to understand*what*a Haskell program calculates, thanks to a concept called**denotational semantics**\. We cover the important notion of a**strict**function, which shows up in many discussions about optimizing Haskell programs\. Lazy evaluation controls the*how*, but not the*what*of a Haskell program\. Additionally, tools for reasoning about space and time usage are provided in the following articles: - [Space invariants — Reasoning about space usage](http://apfelmus.nfshost.com/blog/2013/08/21-space-invariants.html) - [Debit Method — Reasoning about running times](http://apfelmus.nfshost.com/articles/debit-method.html)