F# performance types

Agents write F# collection code that makes several passes, allocates on each element, or uses a tree where a hash table fits. The compiler accepts this code. Human reviewers then ask for changes. Review evidence ranks collection efficiency among the strongest categories of human comments on agent F# code.

For readable shape (records, match, no let mutable accumulators), use fsharp-idiom-review. This skill does not repeat it. Use this skill only for code that a measurement shows to be hot, or that a reviewer or task names as hot. Outside a hot path, prefer the readable form.

Rules

Collections. references/COLLECTIONS.md has the choice table and examples.

• Iterate an F# list with for, List.fold, or pattern matching. Do not index it, and do not copy it to an array only to loop over it.
• Do not enumerate a seq more than once. Each enumeration runs the source again. Materialize it once, or compute all results in one pass.
• Do not use @ or List.append in a loop. Cons onto the head and call List.rev once, or fill a ResizeArray.
• In a hot loop that updates a table, use a local Dictionary or HashSet. Map.add allocates a new tree path on each update, and Map.tryFind allocates a Some on each hit. Return an immutable result at the end if the contract requires one. A Dictionary uses .NET equality: pass HashIdentity.Structural when the key needs F# structural equality, for example an array key.
• Keep the order that the contract defines. List.sort, Array.sort, Map, and Set order strings by ordinal. Array.Sort, List<T>.Sort, and SortedDictionary use the current culture unless you pass StringComparer.Ordinal.
• For a lookup table that is built once and read many times, use FrozenDictionary or FrozenSet.