Foldable
Foldable is a typeclass abstraction for data structures whose elements can be traversed and reduced to a result by folding operations.
Functional Programming in Scala, Second Edition
A foldable data type is a data type whose elements can be traversed and reduced to a result by a folding operation.
- How it is used
- Abstracts reductions over different container shapes through foldLeft, foldRight, foldMap, and combineAll.
- How deep it goes
- Derives fold operations, compares implementation choices, and implements instances for List, IndexedSeq, LazyList, Tree, and Option.
- Examples
- Uses integer summation, List, IndexedSeq, LazyList, Tree, Option, conversion to List, and frequency-map construction.
- What you can do after
- Build it from scratch by defining and optimizing Foldable instances and deriving reusable reductions.
Read how Functional Programming in Scala, Second Edition explains foldable →
Learn Haskell by Example
Foldable is a Haskell type class for data structures whose elements can be traversed and reduced by folding operations.
- How it is used
- Generalizes list reduction across traversable data structures and supplies convenient aggregate operations.
- How deep it goes
- Defines fold directions, gives recursive implementations, and stops short of showing the binary-tree instance code.
- Examples
- Uses candy trails, fruity-candy collection, last-five collection, binary trees, and expression-building folds.
- What you can do after
- Build it from scratch by implementing fold-based helpers and a Foldable instance for a binary tree.
Related concepts
- type classkind of · A type class is a language feature defining operations and constraints that types can satisfy to support ad hoc polymorphism.
- functorcontrasts with · A functor is an abstraction that maps values or objects while preserving the structure and relationships of their surrounding context.
- traverseincludes · Traverse is an operation that maps an effectful function across a traversable structure and collects the results within an applicative context.
- right folduses · A right fold combines the elements of a structure from right to left with an operation and an initial value.
- type constructoruses · A type constructor is a type-level function that takes types as parameters and produces a parameterized type.
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