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Fn(), Functional Programming for Golang

Go Report Card PkgGoDev

Fn is library for golang that enable you to blend functional programming techniques with standard idiomatic Go code.

We are inspired by Clojure, Vavr, and the Java Streams APIs that were introduced back in Java 8, and want to provide something of similar spirit that makes it even more fun to write Go code.


  • Be pragmatic. Fn() is not idealistic, academic, or purely functional. The aim is blend cleanly in to existing Go programs, not to invent a new paradigm for Go. While most of Fn() is built on immutable functional data structures, we live in the mutable world of Go. We try to blend in smoothly instead of ice skating uphill.
  • Be small and concise. Fn() will not come with a tonne of data structures that you might expect from a full-featured functional library. The core Fn() library does not come with immutable red-black tree structures or persistent hash tables, fx.[1]
  • No dependencies. Only the Go standard library.
  • Include simple affordances to interop with constructs from the Go standard library, but no big new frameworking for doing IO or other stuff.

[1]: Some of the more advanced functional data types, like the ones mentioned, are definitely super useful and would fit well in some extension library for Fn(). (or perhaps a sub-package, let's see <3)


For starters let's get some terminology in place.

Seq: The core data structure is Seq[T]. It is short for Sequence. The Seq API is designed to work on top of immutable structures, thus there is no stateful "iterator". Walking through a Seq is done similarly to how you append() elements to a slice in Go, but inversely.

ints = append(ints, i)
// OR if we call the slice "tail" and the element "head":
tail = append(tail, head)

The "inverse" of this operation looks like:

head, tail = tail.First() // pops the first element and returns a new tail

There are many easier ways to walk a Seq though. For example via sq.ForEach() , sq.Take(), and seq.Reduce(). See Iterating Over a Seq.

Generally seqs are immutable. Any exception to this will be clearly documented.

Check the interface definition for Seq here. There many ways to create seqs from standard Go structures. You can find most of them in the section Creating Seqs.

Slice: Standard Go slices []T are wrapped in the seq.Slice[T] type. The Slice type is a public subtype of []T so you can do numeric indexing on a Slice, and use cap(), len(), and for-range loops on them. Slices are seqs, but also add some extra methods like Sort() and Reverse().

Tuple: Or "pair". Represents two data points. A helper mainly used when working with maps, where the tuple captures a key and a value. Maps can be interpreted as a seq of tuples, or if you build a seq of tuples you can create map from it.

Opt: Returned from operations where you are not certain to get a result. For example when you call sq.First(). If the seq is empty you get back an empty opt, and an empty tail sq.

sq.Len(): Lengths are handled in a special way in Fn(). They are allowed to be finite, unknown, or infinite. Making these distinctions opens the possibility of pre-allocating slices and maps of the correct size, which can make a big difference in performance critical code. In most circumstances you will not need to use Len(). All operations are valid on empty seqs and empty opts, so just set up your pipeline of operations and check if the end result is valid.

API Overview

If you just want to jump in and see some code you can check out the simple examples. Otherwise here follows a brief overview.

Fn also bundles a very simple sub-library called 'slice' that you can use to do 1-line functional constructs.

Creating Seqs

We follow the convention that functions for creating a Seq are named with an "Of"-suffix. Ie StringOf(), SliceOf etc. They always return a Seq[T]. Functions with an "As"-suffix return a specific Seq implementation that allows you to perform type specific operations. Fx. like sorting a Slice. It is a known limitation of the Go compiler (v1.19) that it can not determine that generic structs implement generic interfaces. So in order to use a Slice, Map, Set, or String as a Seq you need to call .Seq() on the instance. Seq creation funcs that take a variadic list of arguments have an "OfArgs"-suffix.

From Standard Go Types

arr := seq.SliceOfArgs(1,2,3) // also: SliceOf(), SliceAs(), SliceAsArgs()
ass := seq.MapOf(map[string]int{"foo": 27, "bar": 68}) // also: MapAs()
set := seq.SetOf(map[string]struct{}{"foo", {}, "bar": {}}) // also: SetAs()
str := seq.StringOf("hello world") // also: StringAs()
ch := seq.ChanOf(make(chan T))
twentySeven := seq.SingletOf(27) // single element Seq
empty := seq.Empty[int]()

Numeric Ranges

zero := seq.Constant(0) // infinite
nums := seq.RangeOf(0, 10)
evenNums := seq.RangeStepOf(0, 10, 2)
toInfinity := seq.RangeFrom(0) // "infinity" == max value for the numeric type 

From Functions or Closures

src := seq.SourceOf(func T { ... }) // infinite

Iterating over a Seq

Functions that execute the Seq, ie actively traverse it include:

sq.ForEach(func(elem T) {
   // use elem
sq.ForEachIndex(func (i int, elem T) {
   // use index and elem
tenFirst, tailSeq := sq.Take(10)
goodArray, tailSeq := sq.TakeWhile(func(elem T) bool { return isGood(elem)})

There are also some helper functions included in Fn for executing a Seq for various purposes. See the Operations on Seqs.

Functions that do not execute the Seq, but return a new lazy Seq include:

while := sq.While(predicate)
where := sq.Where(predicate)
mappedSameType := sq.Map(func (val T) T { ... }) // the Seq method Map() can only produce a seq of the same type
mappedOtherType := seq.MappingOf(seq, func(val T) S {}) // becomes a Seq[S]

Transforming Seqs

Limiting the elements seen in a Seq is done with:

sq.TakeWhile(predicate) // if you also need the tail

Transforming elements, mapping them 1-1 is done with

sq.Map(func(t T) T { ... })
seqT := seq.MappingOf(seqS, func(s S) T { ... })

You can split a Seq[T] into sub-seqs with

subs := seq.SplitOf(seq, splitterFunc)

and you can join seqs together with

longSeq := seq.ConcatOf(seq1, seq2, ... )
longSeq := seq.FlattenOf(seqOfSeqs)
longSeq := seq.Prepend(value, seq1) // prepends a single value to a Seq

If you have 2 seqs that you want to traverse in parallel as tuples (pairs) of elements you can use ZipOf:

ints := seq.SliceOfArgs(1,2,3)
strs := seq.SliceOfArgs("one", "two", "three")
pairs := seq.ZipOf(ints, strs)
// pairs is a Seq[Tuple[int,string]]


Predicates that can be used directly on any ordered type T:

seq.IsZero[T] // matching the zero value of a type T 
seq.IsNonZero[T] // matching any non-zero value of a type T
seq.GreaterThanZero[T] // > zero values for T
seq.LessThanZero[T] // < zero value for T

Functions that can help you create a predicate:

seq.Is(x) // val == x
seq.IsNot(x) // val != x
seq.Not(pred) // !pred(val)
seq.GreaterThan(x) // val > x
seq.LessThan(x) // val < x

Collecting Results

The simplest way to collect results from a Seq is to call sq.Values(). It is often desirable to collect the elements into another structure that is not just a slice. Maybe some sort of map, buffer, or completely custom data type.

To this end Fn has the functions Reduce(). This is also known as "fold" in functional programming terminology.

Building a string with Reduce()

strs := seq.SliceOfArgs("one", "two")
res := seq.Reduce(nil, seq.MakeString, strs)
// res is an Opt[string] with the value "onetwo"

Collector Functions For seq.Reduce()

The first argument to Reduce() is a collector function. Fn ships with a suite of standard collectors in the seq package, including: Append, MakeMap, MakeSet, MakeString, MakeBytes, fnmath.Sum, Count, fnmath.Min, fnmath.Max, and GroupBy. There are 2 more advanced collection helpers UpdateMap, UpdateSlice.

Building a Map with MakeMap and TupleWithKey

In order to use MakeMap to build a map we need a Seq of seq.Tuple. The 2 easiest ways to obtain a Seq of tuples are via mapping your seq with TupleWithKey, or via ZipOf.

This example uses TupleWithKey on a *User to build a Seq of Tuple[UserID, *User] and collect that into a map[UserID]*User:

type UserID uint64
type User struct { ID UserID ... }
usersSlice := []*User { ... }

users := seq.SliceOf(usersSlice)
userTuples := seq.MappingOf(users, seq.TupleWithKey(u *User) UserID {
   return u.ID
usersByID := seq.Reduce(nil, seq.MakeMap, userTuples).Or(nil)
// usersByID is a map[UserID]*User, the '.Or(nil)' above converts the Opt result to nil if there are errors

Counting Unique Names with UpdateMap

UpdateMap and UpdateSlice can be used in conjunction with an updater function to create a collector. The updater function tells the collector what to do if there is an existing value in a slot.

In this example we count the number of occurrences of names in a seq. We do this by mapping to names onto a seq of {name, 1} tuples and then instructing the UpdateMap to sum the values every time it merges an element into the map:

names := seq.SliceOfArgs("bob", "alan", "bob", "scotty", "bob", "alan")
tups := seq.ZipOf[string, int](names, seq.Constant(1))
res := seq.Reduce(nil, seq.UpdateMap[string, int](seq.Sum[int]), tups)
// res is an Opt[map[string,int]] with the value:
// map[string]int{
//   "bob":    3,
//   "alan":   2,
//   "scotty": 1,
// }

Operations on Seqs

To check if a Seq contains some given element you can use seq.Any(sq, pred):

nums := seq.RangeOf(0, 10)
hasEvenNum := seq.Any(nums, func (n int) bool { return n % 2 == 0}) // true
hasSeven := seq.Any(nums, seq.Is(7)) // true
allNonZero := seq.All(nums, seq.IsNonZero[int]) // false
// Note: Even if all 3 calls to Any, Any, and All above execute the nums seq
//       this all still work because range-seqs are stateless.

You can also check if all elements satisfy some criteria with seq.All(sq, pred).

Similar to how you can retrieve the first element in a Seq with head, tail := sq.First() you can get the last element with last := seq.Last(sq).

You can check if a seq is empty with seq.IsEmpty(sq).

Executing a Seq for side effects, fx. printing all elements, can be done with seq.Do():

nums := seq.RangeOf(0, 10).
   Map(func (n int) int {
      return n

// Nothing is printed since 'nums' is lazy.
// We can force it to execute with:
// prints numbers from [0..9]


Some operations return Opt[T], notably sq.First() and seq.Reduce(). Opts are used to represent a value that might not be there (if the seq is empty), or capture potential errors. An opt with a captured error is considered empty, and empty opts will report the error opt.ErrEmpty.

They have a range of helper API that allows for easy chaining:

op.Must()   // Returns T or panics if the opt is empty
op.Empty()  // True if there was an error or no value is captured
op.Ok()     // Opposite of Empty()
op.Return() // Unpacks into standard "T, error" pair
op.Or(val)  // Returns val if opt is empty, or the option's own value if non-empty
op.Map(func(val T) T { ... }) // Converts the value to something of the same type
op.Error()  // Returns nil, opt.ErrEmpty, or any captured error
op.OnErr(func (error) T { ... }) // Returns the opt value T, or invokes a callback with the error

// And the static function:
opt.Map(opt, func(val S) T) Opt[T]  // Converts the opt from type S to T

// For async results
opt.Promise(...) Future[T]

Opt Misconceptions and Pitfalls: Opts should be passed by value, on the stack. If you use pointers, *Opt or &Opt, something is wrong. They have a bit of memory overhead and should not be stored in arrays or slices. You can use them in seqs because they are lazily created 1 by 1 and kept short-lived on the stack. An Opt is not a "promise" or "future" - they capture an existing result. If you need async operations please look at opt.Promise().

Parallel Execution

You can execute a Seq in N goroutines mapping the results into a new Seq with seq.Go():

func fetchItem(id int) Opt[T] {
   // do something slow and calculate t
   return seq.OptOf(t) // or maybe an error

// Execute fetchItem of 1027 ids in 100 parallel goroutines
ids := seq.RangeOf(0, 1027)
result := seq.Go(ids, 100, fetchItem)

// result is a Seq[Opt[T]], let's print the successes and errors 
result.ForEach(func (opt Opt[T]) {
   t, err := opt.Error()
   if err != nil {
      fmt.Println("Oh no, an error!", err)
   } else {
      fmt.Println("Nice, got one T:", t)

Error Handling

When operating on in-memory structures like slices, maps, channels and so forth error handling is normally not relevant. But if you do IO or some other operation that can error on runtime Fn provides a few ways to handle it.

The seq.Error(seq) function returns an error if there is an error associated with a Seq or Opt. When you execute a Seq the "empty" tail Seq from ForEach() and other operations will capture any errors.

Alternatively you can wrap results in Opt[T] which can also capture an error. Any error encountered via sq.First() or seq.Reduce() are reported via opts.

The slice Package

Fn includes a package called 'slice' that works directly on standard Go slices and maps and does not use seqs at all. The functions in 'slice' are intended for doing one-shot conversions and mapping elements 1-1.

You can find a few examples of how to use 'slice' in the "examples" folder.

Tips and Tricks

Don't Check Length or Presence Until the Last step

All of the code in Fn() works with nil slices and maps, and empty Opts.

For example, to ensure that there is one and only one record with a given ID in the 10 first records in some slice:

recordID := 1234
recSeq := seq.SliceOf(records).
    Where(func(rec *Record) bool { return rec.ID == recordID})

theOneRecord, err := seq.One(recSeq).Return()

Note how we are not checking the length of 'records', or how many records with the given ID we found. That is all handled by seq.One().

Chan, Map, Set, Slice and String Can be Used As Their Native Go Types

All the seq constructors names with the "As" suffix return their type wrapper.

myMap := seq.MapAs(make(map[string]int))

// We can do normal map[string]int stuff
myMap["one"] = 1
myMapLen := len(myMap)
for k, v := range myMap { ... }

// But also treat it as a seq
result := myMap.Where(...).ToSlice()

Working with Functions that Return Errors

It is very common in Go to have function that look like func getInt() (int, error) or func calcInt(n int) (int, error). This can make function chaining clumsy because you cannot pass the results directly into another function. In Fn this can helped out with opts. Functions that return errors can be wrapped as functions returning opts:

caller := opt.Caller(getInt) // is a func() Opt[int]
mapper := opt.Mapper(calcInt) // is a func(int) Opt[int]

These functions caller, and mapper, can be plugged directly into seq.MappingOf(), seq.SourceOf(), and many others.

Or if you want to calculate the result immediately

optInt1 := opt.Call(getInt)
optInt2 := opt.Apply(calcInt, 27)

There are panic-recovering variations of these functions as well.


If the foundational functional data structures and algorithms is not done carefully, execution speed and memory usage will suffer. Fn() is designed to make the best of what the Go runtime can provide. Initial benchmarks puts it as a top contender among Golang functional libraries. See benchmarks here mariomac/go-stream-benchmarks#1

Experimental Packages

  • seqio provides Seq[[]byte] based on io.Reader, and a Scanner based on bufio.Scanner
  • seqjson provides Seq[T] based on json.Decoder
  • fntesting contains various utilities to test Seqs


* Feels weird that ForEach and ForEachIndex returns an empty seq. Maybe just error, or an Opt?
* Rename seq.MakeXXX into seq.IntoXXX ??

* Something for context.Context? Support cancel() cb and Done() chans? fncontext package...
* seqio.DirOf(dirName), seqio.DirTreeOf(dirName) (recursive)
* RunesOf(string) Seq[rune]
* MakeChan collector func for Reduce()?
* MultiChan() Seq that selects on multiple chan T?
* MergeSort[T any](FuncLess[T], seqs ... Seq[T]) Seq[T] -- lazy merge sorting of pre-sorted Seqs
* Compound FuncCollect, CollectorOf[S,T any](funcs ... FuncCollect[S,T]) FuncCollect[S,[]T]
* Seq[Arithmetic] producing random numbers (in fnmath)?
* Seq for *sql.Rows, with some type safe mechanism for reading rows
* Some kind of "push seq", or is that just Chan? Some libraries only provide "callback based iteration" for data structures.

* EmptySeq impl. (currently just wraps an empty slice), but an empty struct{} would do even better
* Look for allocating buffers of right size where we can
* Can we do some clever allocations in seq.Reduce() when seed is nil?