Operator catalog

Every operator, grouped the way the source is. Full signatures are on pkg.go.dev; this page is for finding the right operator and knowing its costs at a glance.

Markers: [S] carries per-iteration state · ⚠ mem buffers beyond O(1), with its bound · ⚠∞ fully drains — hangs on infinite input · ◐ conditional drain — terminates only when the answer arrives.

Constructors

Operator	Notes
Of, FromSlice	re-iterable; the slice is not copied
From, From2, FromErrs	adapt any push-function iterator, free
FromMap	re-iterable; map order
FromChan(ctx, ch)	single-use; no goroutine; stops on close or cancel
Empty, Empty2, EmptyTry, Once1	the trivial sequences
Repeat, RepeatN	infinite / n copies
Generate, GenerateWhile	seed first, always; infinite / until false
Range(start, stop, step)	overflow-guarded; sign mismatch → empty; step 0 panics
Cycle	infinite; ⚠ mem all (replay buffer); empty input terminates

Type-preserving intermediates (seq.go)

Operator	Notes
Filter, FilterNot, FilterIndexed
Take, Drop	[S] consume exactly what they emit / skip
TakeWhile, DropWhile	[S]
TakeLast(n)	[S] ⚠ mem n, ⚠∞ drains before emitting
DropLast(n)	[S] ⚠ mem n; streams with an n-element lag
Step(n)	[S] first element, then every nth
OnEach	side effects, passthrough
Concat, Append, Prepend
Intersperse(sep)	[S]
IfEmpty(defaults...)	[S] defaults only if the source yields nothing
SortedWith	⚠ mem all, ⚠∞; stable
DistinctWith	[S] ⚠ mem distinct; O(n²) compares — small inputs
Reversed	⚠ mem all, ⚠∞
Once	panics on second consumption — a development guard
UntilDone(ctx)	→ Try; yields ctx.Err() and stops on cancellation

Type-changing intermediates (seq_map.go)

Operator	Notes
Map, MapIndexed
FilterMap	fused filter+map, comma-ok
FlatMap, FlatMapSlice
Scan(init, f)	[S] running fold; init itself is not yielded
WithIndex	→ Seq2[int, T], from 0
ZipWithNext	→ Seq2[T, T]; empty/single input → empty
Zip(other)	→ Seq2; stops at the shorter side; other via iter.Pull
MapErr, FilterErr	→ Try; a failed callback yields (zero, err)
DistinctBy(sel)	[S] ⚠ mem distinct keys; first wins
DedupeBy(sel)	[S] O(1); consecutive duplicates only
SortedBy, SortedByDesc	⚠ mem all, ⚠∞; stable; sel once per element
JoinBy	inner hash join; ⚠ mem right side, buffered before first emission

Consumption and search terminals (seq_terminal.go)

Operator	Notes
Collect, ToList	⚠∞; nil for empty
Seq()	free conversion to iter.Seq
Pull()	caller MUST call stop, or the producer’s cleanup never runs
ToChan(ctx)	unbuffered; goroutine ends on drain or cancel
ForEach, ForEachIndexed, Drain	⚠∞
ForEachErr	◐ stops at the first error f returns
First
Last, FindLast	⚠∞
Single	◐ true iff exactly one; stops at the second element
ElementAt(i)	negative i → (zero, false), consumes nothing
Find, FindIndex, FindMap	◐
Any, All, None	◐; All vacuously true on empty
Count, CountWhere	⚠∞
IsEmpty	consumes ONE element — lost on a single-pass source

Folds and grouping (seq_fold.go)

All ⚠∞; map results iterate in undefined order.

Operator	Notes
Fold, FoldIndexed
FoldWhile	◐ the stopping call’s accumulator is included
FoldErr	◐ accumulator-so-far + first error
FoldBy(key, init, f)	⚠ mem distinct keys — the streaming GroupBy
Reduce	first element as initial accumulator; (zero, false) on empty
GroupBy	⚠ mem all; buckets in encounter order
IndexBy, Associate	last wins on key collisions
TallyBy	counts per key
Partition	both sides in encounter order

Aggregation (seq_aggregate.go)

All ⚠∞; cmp.Compare ordering (NaN below everything); ties first-wins; (zero, false) on empty.

Operator	Notes
MaxBy, MinBy	the element with the extreme key
MaxOf, MinOf, MinMaxOf	the extreme key; MinMaxOf in one pass
MaxWith, MinWith	comparator forms
TopNBy, BottomNBy	⚠ mem n; sorted output; stable at the cut
SumOf, ProductOf	wraps like +; empty product is 1
AverageOf	float64 accumulation; (0, false) on empty
JoinToString

Package functions (funcs.go)

Constraint-bound — these cannot be methods because they require something of T.

Operator	Notes
Distinct, Dedupe	[S]; first wins / consecutive only
Sorted, SortedDesc	⚠ mem all, ⚠∞; stable
Sum, Product, Average, Max, Min, MinMax, TopN	⚠∞
Contains, IndexOf	◐
NonZero	drops zero values
ToKeySet, Tally, AssociateWith	⚠∞
Equal	consumes both to the first difference; b via iter.Pull
Union, Intersect, Except	set semantics, encounter order; ⚠ mem
Flatten, FlattenSlices, Chain
Unzip, CollectMap, Join	⚠∞

Batch operators (batch.go)

Package functions of necessity — a Seq[T] method returning Seq[[]T] is an instantiation cycle. Every emitted slice is fresh; retaining one is always safe.

Operator	Notes
Chunked(s, n)	[S]; last chunk may be partial
ChunkedBy(s, sel)	[S] ⚠ mem longest run; a chunk per run of equal keys
Windowed(s, size, step)	[S] ⚠ mem size; full windows only; step > size samples

Seq2, Try, List

Seq2 carries the bridge set: Filter, Map, MapValues, MapTo, Take, Drop, Keys, Values, Swap, Fold, ForEach, Any, All, Count, First, Seq2, Pull. To a map: catena.CollectMap. To two slices: catena.Unzip.

Try mirrors the intermediates under the five error rules (03-error-handling.md) and adds the policy terminals: Collect, CollectAll, Ignore, Errs, Must, Err, plus Recover, WrapErr, OnError, UntilDone.

List mirrors the whole Seq operation set eagerly (generated, and conformance-checked to agree with AsSeq()), plus Len, At, Get, Slice, Clone, AsSeq, FoldRight, and a never-aliasing Append.