JetBrains MPS is a development environment for creating domain-specific languages, which are programming languages designed for a particular field or task. It provides editors with features such as completion, semantic checks, and type checking, and can generate code in languages including Java and XML.
Borrowing it
Nothing to install: this file belongs to JetBrains/MPS. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/JetBrains/MPS/master/.agents/skills/mps-collections-and-closures/SKILL.mdgit clone --depth 1 https://github.com/JetBrains/MPSWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/jetbrains/mps/mps-collections-and-closures)<a href="https://agentmods.dev/skills/jetbrains/mps/mps-collections-and-closures"><img src="https://agentmods.dev/badge/skills/jetbrains/mps/mps-collections-and-closures/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/jetbrains/mps/mps-collections-and-closures"><img src="https://agentmods.dev/badge/skills/jetbrains/mps/mps-collections-and-closures.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00225 | $0.02039 |
| Opus 5.5 | $0.00090 | $0.00816 |
| Sonnet 5.5 | $0.00045 | $0.00408 |
| Haiku 4.5 | $0.00022 | $0.00204 |
Grade A, and why
mps-collections-and-closures scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 29d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 68 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Writing MPS BaseLanguage with collections / closures
Loading companion skills
Companion names in this skill are lazy dependencies: load only those relevant to the current task. If this skill came from an MCP server, use the host's skill loader to resolve the companion's unique discovered entry URI on the same host-assigned originating server. If the host has no server-backed skill loader, stop and report that limitation; do not silently fall back to a filesystem copy. If this skill came from a filesystem catalog, load the named sibling from that same catalog at <skills-root>/<skill-name>/SKILL.md, even if remote skill loaders are also available. Do not invent a tool name or server endpoint.
Use this skill when a body filters, maps, sorts or iterates a sequence, or passes a closure — to a collection operation, a Java API (SAM conversion), or a variable of function type. A collection operation is a DotExpression whose operation is a collections *Operation concept with a single ClosureLiteral in the closure role. The mps_mcp_parse_java_and_insert parser maps Java lambdas to closures but cannot produce collection types or operations, so these shapes are hand-built from the blueprints here.
Minimum reading set
references/ is a lookup catalog. One file is enough to write the body.
| Body you are writing | Read only |
|---|---|
filters or maps a sequence (.where, .any, .translate, list.add) |
references/sequence-operation-blueprints.md |
| a closure that does not type-check | references/closures-catalog.md |
an iteration that fails (foreach) |
references/foreach-statements.md |
crosses into a Java interface (Iterable.iterator(), Map.computeIfAbsent()) |
references/downcast-expression.md |
| an operation whose name or shape you do not know | references/collections-catalog.md |
Stopping rule. After an error you cannot explain, open references/golden-rules-and-pitfalls.md before any catalog.
Critical Directives
What ships with it
15 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
- references/closures-catalog.md 9.3 KB
- references/collections-catalog.md 1.4 KB
- references/collections-catalog/concept-ids.md 368 B
- references/collections-catalog/control-flow-in-closures.md 553 B
- references/collections-catalog/core-concepts-types-null-semantics.md 2.9 KB
- references/collections-catalog/creators.md 654 B
- references/collections-catalog/iterators.md 865 B
- references/collections-catalog/list-mutators.md 904 B
- references/collections-catalog/sequence-operations.md 4.5 KB
- references/collections-catalog/set-and-map-operations.md 1.2 KB
- references/collections-catalog/sorted-collections.md 1.1 KB
- references/downcast-expression.md 4.4 KB
- references/foreach-statements.md 3.0 KB
- references/golden-rules-and-pitfalls.md 3.7 KB
- references/sequence-operation-blueprints.md 4.9 KB
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 29d ago First seen · 68 lines · 225 tokens per session scan A a387fb235946
mps-collections-and-closures is a skill published in the GitHub repository JetBrains/MPS (1,666 stars, last pushed yesterday), licensed Apache-2.0. It adds 225 tokens to every session and 2,039 once invoked, about $0.0009 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-10-09.
mps-collections-and-closures compared with skills from other repositories
| Mod | Type | Stars | Last push | Licence | Scan | Tokens | Works with |
|---|---|---|---|---|---|---|---|
| mps-collections-and-closuresJetBrains/MPS | Skill | 1.7k | yesterday | Apache-2.0 | A | 225 | Claude Code, Codex |
| langiumeclipse-langium/langium-ai | Skill | 31 | 4d ago | MIT | A | 33 | Claude Code |
| lai-gen-descriptoreclipse-langium/langium-ai | Skill | 31 | 4d ago | MIT | A | 49 | Claude Code |
| lai-gen-mcpeclipse-langium/langium-ai | Skill | 31 | 4d ago | MIT | A | 43 | Claude Code |
| lai-gen-language-skilleclipse-langium/langium-ai | Skill | 31 | 4d ago | MIT | A | 33 | Claude Code |
| triton-syntaxmindspore-ai/akg | Skill | 265 | 2d ago | Apache-2.0 | A | 21 | Claude Code, Codex |
| triton-ascendmindspore-ai/akg | Skill | 265 | 2d ago | Apache-2.0 | A | 28 | Claude Code, Codex |