# std.collections > Small pure list and string helpers used across the Sema standard library — joins, stable rounding, and order-preserving de-duplication. `std.collections` is the standard library's own toolbox of **small, pure helpers** for lists and strings. Every function here has an empty effect row `!{}` — they compute and nothing else — which is why the rest of the stdlib (and `document` in particular) builds on them. ```sema from std.collections import join_str, join_ints, join_floats, dedup_ints, r6 ``` ## Why it exists These are the little utilities that would otherwise be re-inlined in every module: joining a list into a delimited string, rounding to a stable number of decimals for display, de-duplicating while preserving order. Centralizing them keeps the rest of the stdlib compact and gives your own deterministic core the same building blocks. For the primitive collection types (`list`, `dict`, `tuple`) themselves, see [Types](/language/types/). ## String joins `join_str` joins a list of strings with a separator. `join_ints` and `join_floats` stringify first (floats via `r6`, below): ```sema join_str(["a", "b", "c"], ", ") # "a, b, c" join_ints([1, 2, 3], "-") # "1-2-3" join_floats([0.1, 0.25], " | ") # "0.1 | 0.25" ``` ## Stable numeric display `r6` rounds to six decimal places, so numeric output is stable and comparable across runs (no long floating-point tails): ```sema r6(0.12345678) # 0.123457 ``` `join_floats` uses `r6` internally, so a list of floats renders consistently. ## Order-preserving de-duplication `dedup_ints` removes duplicate integers while keeping first-seen order — useful for collapsing a list of ids without sorting it: ```sema dedup_ints([3, 1, 3, 2, 1]) # [3, 1, 2] ``` :::note[The pure-core discipline] Every helper here carries `!{}`. Keeping utility code in the deterministic, invoke-free core means it is trivially testable and safe to call from anywhere — including inside a `simulate def` contract or a metered scope. ::: :::tip[Semantic de-duplication] `dedup_ints` is exact equality over ints. When you need to collapse *near* duplicates — paraphrases, restatements — reach for the neurosymbolic `semantic.dedup` operation instead, which uses calibrated similarity. ::: See the full signature list in the generated [std.collections API reference](/reference/stdlib-api/collections/).