Fmap
Fmap is an operation that applies a function to values within a functor while preserving the functor’s structure.
Learn Haskell by Example
fmap is the standard Haskell function that applies a function to the contents of a value in a Functor while preserving the surrounding structure.
- How it is used
- Transforms parsed, command-line, and generated values without discarding their `Either`, `IO`, `Parser`, or generator context.
- How deep it goes
- Defines the Functor type and kinds, then implements a `Parser` instance through nested `Either` and tuple mappings.
- Examples
- Demonstrates CSV construction, `String`-to-`Text` command arguments, magic-number parsers, and a proposed `RandomIO` Functor instance.
- What you can do after
- Apply it via `fmap`, `<$>`, and `<$` to transform successful or produced values while preserving their wrapper.
Functional Programming in JavaScript
A mapping operation for a functor that applies a function to the wrapped value and returns a new functor of the same type.
- How it is used
- Uses functor mapping to transform encapsulated values while preserving their wrapper context.
- How deep it goes
- Defines fmap, implements it in Wrapper, and demonstrates chained transformations, but does not develop formal functor laws.
- Examples
- Demonstrates Wrapper values, incrementing 2 to 5, chaining to 15, identity extraction, and logging with tap.
- What you can do after
- Build it from scratch by implementing Wrapper.fmap and chain transformations without directly accessing the wrapped value.
Read how Functional Programming in JavaScript explains fmap →
Functional Design and Architecture: Examples in Haskell
fmap is the standard functor-mapping operation that applies a function to a functor's contained values while preserving its structure.
- How it is used
- Uses one abstract operation to transform values across lists, Maybe values, and free-monad structures without changing their shapes.
- How deep it goes
- Defines Functor behavior, works through list and Maybe examples, and implements fmap for a custom free-monad constructor.
- Examples
- Uses name-length lists, nested Maybe values, integer transformations, and SandwichConstructor continuations.
- What you can do after
- Apply transformations with fmap while preserving list, Maybe, or custom Functor structure.
Read how Functional Design and Architecture: Examples in Haskell explains fmap →
Related concepts
- (:>)equivalent to · (:>) is a Servant type operator that combines route or request information into the description of one API operation.
- bindused by · Bind is a monadic operation that sequences a context-bearing computation with a function producing the next computation while flattening the resulting nested context.
- mapequivalent to · An operation that applies a function to values in a collection or computational context, producing corresponding results while preserving the collection or context’s structure.
- functorpart of · A functor is an abstraction that maps values or objects while preserving the structure and relationships of their surrounding context.
- functionuses · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
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