As programmers we rarely have a luxury to write a project from scratch. Usually we have to retrofit existing projects with all cool things we need. If a new component, or a library we want to use introduces new concepts that bleed outside its boundary, we have a “culture clash”, when old code is unaware about new concepts have to work with it anyhow. Sometimes the clash is so bad that we have to give up on using shiny new things, or have to significantly rework their code, which requires time and efforts we cannot afford.

Incomplete objects allow us to concentrate on important properties of JavaScript objects ignoring the rest: we don’t need to specify every single property, and we can deal with cyclical graphs. Incomplete arrays is a complimentary feature to inspect only the first few array items. Both features are very useful for patterns, and heya-unify provides rich facilities to automate creating incomplete objects: they can be marked up explicitly on per-instance basis, recursively with a special utility, and we can specify how to deal with objects by default during unification.

Custom unification in heya-unify allows us to deal with our specific objects in JavaScript, create unification helpers, and even custom logical conditions. It is there to bridge unification with our existing projects. Looking at the 1st part and the 2nd part of the series is recommended before diving into details. Custom unification Unification makes comparing simple objects a cinch no matter how complex they are, and we can easily apply it to JSON-like trees as is.

Unification for JS introduced heya-unify — a practical mini library to leverage unification in JavaScript. This post explains when it makes sense to use unification, and gives practical examples of how to use it. When to unify? Below is my laundry list for unification. As soon as I see a project, which deals with items on the list, I investigate if it makes sense to use heya-unify. Matching and transforming An obvious sweet spot is when we need to inspect deep objects saving sub-objects for future use, and possibly matching some sub-objects.

Unification is a very interesting programming tool. Originated from logical programming (its the foundation of Prolog) and used in functional programming (e.g., Haskell) it helps to compare objects for equality, identify known patterns, and reassemble results differently. Wikipedia gives a somewhat complex definition of unification, but many people see it as an extended equivalence comparison, a pattern matching tool, and some even find parallels with XPath queries, CSS, and even jQuery, all operating on regular objects.