forcomprehension

Forcomprehension, commonly written as for-comprehension, is a language feature in functional programming languages, most prominently in Scala, that lets developers express operations over one or more monadic or iterable sources in a compact, readable block. It combines multiple generators, optional guards, and value bindings into a single expression and then yields a final result. The construct is designed to abstract away explicit nesting of map and flatMap calls, making code easier to write and maintain when working with collections, options, futures, or other monads.

In a for-comprehension, each line introduces a generator x <- xs, an optional guard if cond, or a binding z = expr. The block ends with yield to produce values. The compiler desugars the for-expression into a chain of map and flatMap calls, withFilter inserted for guards. The result is a monadic composition whose exact form depends on the underlying type.

Example: in Scala, for { x <- List(1, 2, 3); y <- List("a","b") } yield (x, y) yields List((1,"a"), (1,"b"), (2,"a"), (2,"b"), (3,"a"), (3,"b")). Under the hood, this desugars to List(1,2,3).flatMap(x => List("a","b").map(y => (x,y))). For guards such as if x > 1, the translation uses withFilter to skip elements before mapping. Forcomprehensions are commonly used with options, futures, and other monads to compose operations in a concise and expressive way.