Combiners

Combiners are the only arbitraries in fast-check that do not generate anything on their own. They take one or more existing arbitraries as input and return a new one. They are the functional glue that lets a handful of primitives and composites cover the full space of values a real codebase cares about.

You will reach for a combiner whenever you want to:

• promote plain values into the arbitrary world (constant, constantFrom),
• choose between alternatives (oneof, option),
• tie arbitraries together recursively (letrec) to generate trees, ASTs or JSON-like structures,
• refine or transform an existing arbitrary with .filter, .map, .chain, or adjust its shrinking with noShrink / limitShrink.

Because every combiner wraps another arbitrary, shrinking composes too: the combiner preserves (or deliberately limits) the shrink behaviour of the arbitrary it wraps, so a counterexample through oneof(...).map(...) still collapses toward the simple values defined by the innermost primitive.

📄️Any

Combine and enhance any existing arbitraries.

📄️Constant

Promote any set of constant values to arbitraries.

📄️Recursive Structure

Define arbitraries able to generate recursive structures.

📄️String

Combine and enhance existing string arbitraries.