The Builder Pattern in F#

In this post, we’ll look at the “builder pattern”, and discuss various ways of implementing it in F#.

The builder pattern is used when you need to construct a valid object using a series of individual steps, rather than using a single constructor. This need arises when:

• you want to break the construction into steps for clarity. This can be useful when constructing complex objects for testing. It can also be used for performance, such the StringBuilder class in C# and Java.
• you want to validate each construction step separately.
• you don’t have all the data available, e.g. while a user is filling out a multi-part form, or when partial data arrives as events or messages. (This last one is sometimes also called an Aggregator).

In F# there is another concept called “builder” – a computation expression builder. This is not related to the classic “builder pattern” at all. For more, see the computation expressions series.

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