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Tony Hannan 2011-09-12 10:46:24 -04:00
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There are plenty of languages to choose from for your Mongo application, so why should you consider [Haskell](http://www.haskell.org/)? The reason is there are several unique features in Haskell that help reduce bugs and increase programmer productivity and serenity. In this article, I will introduce some of these features and describe how they could benefit your application. I will also discuss one troublesome feature of Haskell that you should be aware of. There are plenty of languages to choose from for your Mongo application, so why should you consider [Haskell](http://www.haskell.org/)? The reason is there are several unique features in Haskell that help reduce bugs and increase programmer productivity and serenity. In this article, I will introduce some of these features and describe how they could benefit your application. I will also discuss one troublesome feature of Haskell that you should be aware of.
Before we get into features, let me address probably the most common reason why many people do not consider Haskell: because Haskell is not popular (no one they know uses it). My rebuttal to this is: even though Haskell is not popular, it is popular *enough*. The Haskell community and ecosystem (libraries and tools) has reached a critical mass such that you can easily find previous success stories (see [Haskell in industry](http://haskell.org/haskellwiki/Haskell_in_industry)), find Haskell programmers to hire (just send an email to the [Haskell mailing list](http://www.haskell.org/haskellwiki/Mailing_Lists)), and find libraries for most anything you would expect to find in a popular language (see [Hackage](http://hackage.haskell.org/packages/archive/pkg-list.html) for the list of over 3000 libraries). Let's define a *mainstream* language as one with this critical mass. I claim that you should consider all mainstream languages (I would say there are about 10 of them). Assuming you agree, let's now judge Haskell on its features alone. Before we get into features, let me address probably the most common reason why many people do not consider Haskell: because Haskell is not popular (no one they know uses it). My rebuttal to this is: even though Haskell is not popular, it is popular *enough*. The Haskell community and ecosystem (libraries and tools) have reached a critical mass such that you can easily find previous success stories (see [Haskell in industry](http://haskell.org/haskellwiki/Haskell_in_industry)), find Haskell programmers to hire (just send an email to the [Haskell mailing list](http://www.haskell.org/haskellwiki/Mailing_Lists)), and find libraries for most anything you would expect to find in a popular language (see [Hackage](http://hackage.haskell.org/packages/archive/pkg-list.html) for the list of over 3000 libraries). Let's define a *mainstream* language as one with this critical mass. I claim that you should consider all mainstream languages (I would say there are about 10 of them). Assuming you agree, let's now judge Haskell on its features alone.
Haskell's most important and distinguishing feature is its [*declared effects*](http://en.wikipedia.org/wiki/Effect_system). No other mainstream language has it. Declared effects means the type of a procedure includes the type of side effects it may have (in addition to the type of value it returns). In other words, the type declares what the procedure *does*, not just its inputs and output. For example, a procedure with type `Document -> DbAction ObjectId` declares that the procedure takes a Document, performs a DbAction, and returns an ObjectId. If you looked at DbAction's type definition it would say that a DbAction reads/writes to the database and may throw a DbException. Haskell's most important and distinguishing feature is its [*declared effects*](http://en.wikipedia.org/wiki/Effect_system). No other mainstream language has it. Declared effects means the type of a procedure includes the type of side effects it may have (in addition to the type of value it returns). In other words, the type declares what the procedure *does*, not just its inputs and output. For example, a procedure with type `Document -> DbAction ObjectId` declares that the procedure takes a Document, performs a DbAction, and returns an ObjectId. If you looked at DbAction's type definition it would say that a DbAction reads/writes to the database and may throw a DbException.