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copied!<p>A "type and effect system" describes not only the kinds of values in a program, but the changes in those values. "Typestate" checking is a related idea.</p> <p>An example might be a type system that tracks file handles: instead of having a function <code>close</code> with return type <code>void</code>, the type system would record the <em>effect</em> of <code>close</code> as disposing of the file resource—any attempt to read from or write to the file after calling <code>close</code> would become a type error.</p> <p>I'm not aware of any type and effect system appearing in a mainstream programming language. They have been used to define static analyses (for example, it's quite natural to define an analysis for proper locking/unlocking in terms of effects). As such, effect systems are usually defined using inference schemes rather than concrete syntax. You could imagine a syntax looking something like</p> <pre><code>File open(String name) [+File]; // open creates a new file handle void close(File f) [-f] ; // close destroys f </code></pre> <p>If you want to learn more, the following papers might be interesting (fair warning: the papers are quite theoretical).</p> <ul> <li><a href="http://slang.soe.ucsc.edu/cormac/papers/atomic-toplas.pdf" rel="noreferrer">Types for Atomicity: Static Checking and Inference for Java</a>. Flanagan, Freund, Lipshin, and Qadeer.</li> <li><a href="http://research.microsoft.com/~maf/Papers/pldi01.pdf" rel="noreferrer">Enforcing High-Level Protocols in Low-Level Software</a>. Robert DeLine and Manuel Fändrich.</li> <li><a href="http://www.cs.ucla.edu/~palsberg/tba/papers/nielson-nielson-csd99.pdf" rel="noreferrer">Type and Effect Systems</a>. Nielson and Nielson.</li> </ul>

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