Conciseness

The body is mostly efficient — type tables, tight code blocks, and genuinely non-obvious notes like the Go type-parameter limitation on Result.Map. It could still be trimmed: the second mo.Do failure example is redundant with the first, the "Think of it as..." analogy column re-explains concepts Claude already knows, and the Pipeline section repeats the sub-package pattern already shown under Result. Efficient with minor over-explanation matches the 4 anchor rather than the lean 5.

Actionability

Nearly all guidance is executable Go with imports, method inventories, and concrete rules of thumb, but a few examples have minor gaps: the Either.Match example has empty function bodies with the required return omitted (won't compile as written) and `ptr` in the Option example is undefined. Mostly executable with minor gaps fits the 4 anchor; the placeholder-heavy Match example keeps it below the fully copy-paste-ready 5.

Workflow Clarity

As a library-usage skill it has no risky multi-step process, and it sequences guidance well (glance table → per-type sections → composition rules → common patterns → numbered best practices) with explicit decision checkpoints such as "Use `Result[T]` for errors, `Either[L, R]` for alternatives" and the direct-methods vs sub-package rule of thumb. There is no explicit validation/feedback loop, but none is required for this non-destructive skill, so this sits at 4 rather than 5.

Progressive Disclosure

SKILL.md is a genuine overview: each type section ends with a clearly signaled one-level-deep link to a real reference file (option.md, result.md, either.md, pipelines.md, advanced-types.md — all present with substantive content), and the monads introduction is split into monads-guide.md. All six referenced paths resolve to actual files with no nested references, matching the 5 anchor exactly.

Specificity

The description names the exact library (samber/mo), enumerates all eight concrete types ("Option, Result, Either, Future, IO, Task, and State"), and states their specific purposes ("type-safe nullable values, error handling, and functional composition with pipeline sub-packages") — comprehensive concrete coverage with no gaps. It stays in third person throughout.

Completeness

It explicitly answers both questions: the "what" (monadic types for nullable values, error handling, and functional composition) and a fully explicit "Apply when..." clause with three concrete trigger conditions. This matches the 5 anchor's pattern of concrete trigger phrases, and the added "Not for..." boundaries further remove ambiguity.

Trigger Term Quality

Natural triggers include "using or adopting samber/mo", "the codebase imports `github.com/samber/mo`", "functional programming patterns", and "Golang" (covering both Go and Golang phrasings). A few natural terms users might say are missing, such as "monads", "Optional", or a .go extension trigger, so it falls just short of the comprehensive-synonym 5 anchor.

Distinctiveness Conflict Risk

The trigger is anchored to a specific library and import path, and it explicitly disambiguates the two nearest competing skills ("Not for nil-safety... → golang-safety", "nor for native error wrapping... → golang-error-handling"), leaving minimal risk of triggering the wrong skill.