PragPub 2012-08: Issue #38 by The Pragmatic Bookshelf

Author:The Pragmatic Bookshelf
Language: eng
Format: epub, mobi
Tags: PragPub—Monthly Magazine
Publisher: The Pragmatic Bookshelf, LLC
Published: 2012-08-01T04:00:00+00:00

A Very Important (and Under-Appreciated) Type

Like most functional languages, Haskell has first-class functions, meaning we can treat functions like almost any other piece of data--build them, pass them around, use them. We should not forget how such functions sit with the data types above. The notation for the type is A -> B, indicating a conversion from some type A to some other type B. Tests on color values will have type RGB -> Bool, or determining the max of two colours with RGB -> RGB -> RGB. Values of these types can appear in record fields, etc., for (a very contrived) example, each person could include a mapping from an int to a color which expresses what colour that person associates with a number.

We can also represent "waiting for an X" as a function value; e.g., if you have a person record but are waiting for their address to come from somewhere, this could be represented as a function of type [String] -> Person which is supplied with the address when it is available, and will return the complete person thereafter. Using the example above, we can do it like this, using Haskell's syntax for anonymous functions:

\address -> Person "Joe" (Just 25) Red address

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