FxDart 101 › Section 10 · Function utilities › cases

cases

Builds a predicate/mapper dispatch table, with an optional default.

Lecture

cases builds a matcher out of a list of (predicate, mapper) pairs: it tries each pair in order, and the first one whose predicate returns true has its mapper applied to produce the result. It's a functional stand-in for a chain of if/else if, and it composes nicely with map — the returned function is a plain unary T -> R you can pass straight in.

This shape differs from FxTS on purpose. FxTS's cases is variadic: each [predicate, mapper] pair is its own trailing argument, with an optional final bare function acting as the default, and TypeScript's overloaded generics type each arity by hand. Dart has neither variadic generics nor per-arity overloads, so there's no way to accept "any number of pair arguments" and still get real type checking. FxDart's version instead takes one List of (predicate, mapper) records — Dart's tuple type — plus a separate named orElse parameter so the default is never confused with just another pair.

If nothing matches and no orElse is given, cases falls back to returning value itself — which only compiles at the call site if T happens to also satisfy R — otherwise it throws a StateError at runtime. In practice, always pass orElse unless you're certain your predicates are exhaustive.

Demo 1 · Basics

Demo 2 · A grading pipeline

Try it yourself

Exercise: classify each size as 'small' (<10), 'medium' (<30), or 'large'.