A FileMaker custom function is a reusable calculation created by a developer for a particular custom app.
FileMaker Pro already includes many built-in functions for working with text, numbers, dates, lists, JSON, and other types of information. Custom functions allow developers to create additional functions adapted to the specific needs of their application.
Once defined, a custom function can be used throughout the FileMaker file in calculations, scripts, field definitions, validation rules, conditional formatting, and many other places.
Turning a Formula into a Reusable Function
Imagine that the same calculation is required in several parts of a custom app. The developer could copy the complete formula into every field and script where it is needed.
A better approach is often to turn that formula into a custom function with a meaningful name.
Instead of repeatedly displaying a long calculation, the application can call a function such as:
FormatCustomerReference ( customerId )
The details of how the customer reference is formatted remain inside the function. The calculations that use it only need to specify what should be processed.
This makes the application easier to understand and maintain.
Centralizing Business Logic
One of the principal benefits of custom functions is the centralization of code.
When the same rule is written separately in ten calculations, changing that rule may require ten modifications. It is also easy to overlook one occurrence or introduce small differences between copies.
With a custom function, the logic is defined in one place. Updating the function automatically updates every calculation that calls it.
Custom functions can therefore help centralize rules such as:
- formatting identifiers and reference numbers;
- normalizing names, telephone numbers, or addresses;
- validating structured information;
- manipulating lists or JSON;
- converting values between formats;
- applying calculations specific to the organization.
This reduces duplication and helps ensure that the same rule is applied consistently throughout the custom app.
Making Calculations Easier to Read
A calculation should communicate its intention, not merely produce the correct result.
Long formulas containing many nested FileMaker functions can become difficult to read. A well-named custom function can hide the implementation details and reveal what the calculation is meant to accomplish.
For example, a developer reading IsValidVATNumber ( vatNumber ) can immediately understand the purpose of the calculation, even without examining the complete validation logic.
This improves readability for the original developer and for anyone who maintains the application later.
Simplifying Complex Calculations
Custom functions allow a complex operation to be divided into smaller, clearly named components.
One function can also call another custom function. Developers can therefore build a library of reusable calculation tools and combine them to solve more complex problems.
This modular approach generally makes calculations easier to:
- understand;
- test;
- debug;
- document;
- reuse;
- modify safely.
The objective is similar to using functions in a conventional programming language: each function should perform a clearly defined operation and return a result.
Recursion
A custom function can call itself. This technique is known as recursion.
Recursion allows a function to repeat an operation until a condition is satisfied. It can be useful for processing lists, navigating structured data, performing repeated transformations, or solving problems whose definition naturally refers back to itself.
Modern versions of FileMaker also provide the While function for many iterative calculations. Nevertheless, recursion remains an important capability of custom functions and may provide an elegant solution for certain problems.
Reusing Functions Between FileMaker Apps
Custom functions belong to the FileMaker file in which they are defined. Once created, they can be used anywhere in that file.
They can also be copied or imported into other FileMaker files. This allows experienced developers to maintain libraries of reliable functions and reuse them across different projects.
However, a function may depend on other custom functions, fields, or elements of the original app. These dependencies must be considered when moving it to another file.
Custom Functions and Maintainable Development
Custom functions do not automatically make an application better. Poorly named or unnecessarily complex functions can make a solution harder to understand.
Used with a consistent naming convention and clear responsibilities, however, they are an important part of professional FileMaker development. They help developers avoid duplicated code, express business rules more clearly, and make future changes safer.
At 1-more-thing (fmcloud.fm), our 30 years of FileMaker experience have taught us that maintainability is just as important as delivering the initial feature. A custom app may remain in production for many years and pass through several developers. Well-designed custom functions help preserve clarity and consistency throughout that lifecycle.
Conclusion
A FileMaker custom function packages a calculation under a reusable name.
Its main advantages are:
- centralizing calculation logic;
- avoiding duplicated formulas;
- simplifying complex calculations;
- improving readability;
- making maintenance easier;
- encouraging consistent business rules;
- enabling recursive processing;
- creating reusable libraries of FileMaker code.
Custom functions are largely invisible to end users, but they can make a significant difference to the quality, reliability, and maintainability of a FileMaker custom app.
- About custom functions
- Using custom functions
- Working with formulas and functions
- A great library of Custom functions: Brian Dunning’s web site.
