Design rationale and open questions
This page explains why Hypermedia Schemas have their shape, and what is still undecided.
Principles
Minimal enough to describe itself. The ten structural keys are the smallest set that can express their own structure. name and description are optional metadata on top. Every candidate feature has to pass one test: the meta-schema must still be a valid instance of itself. The schema language shows the loop, and validate.mjs checks it. Self-description is the design constraint.
Precise. The meta-schema is a discriminated union of closed variants, so it rejects malformed schemas such as {type:"string", items:{…}}. A type system whose own type is loose would accept nonsense. Making schema a union lets it reject that.
Name kinds, don't define them. The codec defines what a string or a link is. The schema language only names codec-defined kinds so schemas can constrain values to them. That is why link and bytes could join with no structural change: they are two more literals in the same union as string.
Representation is separate from kind. A link renders as {"/":…} in JSON, but it is not a map and is never modeled as one. Atomic kinds avoid the reserved-key ambiguity of dag-json. Schemas describe what a value is, not how it is spelled in one encoding.
Decisions taken
integer / float split. DAG-CBOR encodes them differently, and merging them loses round-trip fidelity. They stay distinct even though JSON has one number type. See the data model.
map, not object. The data-model kind is map. A struct is a map constrained by properties. There is no separate object or struct kind.
Typed links via target. type:"link" plus target gives the expected type of what the link points at. Every reference-valued leaf shares that one key: a string field with format: hm-url or ipfs-url carries the same target. Links get no vocabulary of their own. Target checking has to be lazy. See references and link-schema.
References are names (hm:// URLs), not CIDs. A CID cycle has no encoding order. Recursion, including mutual recursion (example/folder and example/file point at each other), can only be expressed with names. Names resolve through a signing authority. The library writes its authority as hyper.media, which the SDK resolves to its bundle until the network resolves domains in hm:// URLs. CIDs pin immutable versions. This replaced an earlier self-sentinel idea. See references and URLs.
type is a URL, and it is the only naming key. type: "hm://hyper.media/map" makes the kind clickable and self-explanatory. Each primitive, such as map, is a self-grounding axiom whose type names itself. The same key names another schema when the node includes or extends one, so naming a kind and naming a type are one act. The validator still reads the kind locally off the URL, so the discriminated union stays local and strict. The older ref key is retired.
Extension by refinement. { type: parent, …refinements } extends the parent as a subtype: properties merge, required unions, and closedness is kept. A bare { type: parent } is a pure include. The rule is uniform: type names, and any other key refines, for structural and leaf schemas alike. So there is no extends keyword and no separate spelling for a leaf that narrows its kind. Example: example/employee extends example/person. See the schema language.
anyOf unions. anyOf was added so schema can be a discriminated union. It is the one composite construct, and it stays self-describing: anyOf is a list of schemas, expressible with the existing vocabulary, and the meta-schema validates as its own union variant. See the schema language.
Literals instead of enum. A value is a schema of itself: "Change" accepts exactly "Change". A fixed set of choices is a union of literals, {anyOf: ["draft", "published"]}. This replaced an enum list on scalar schemas. It removed a keyword, let every choice carry its own description, and made a pinned tag read plainly (type: {value: "Change"}). A literal can only be a string, integer, boolean or null. A map literal would look the same as a schema, and float equality is unreliable. See the schema language.
Closed maps by default. A map with properties and no values rejects unknown keys. This settles the closedness question, and it lets the discriminated union reject malformed schemas. Existing schemas and data stay valid because they list their keys. The cost: data with unknown or future fields is rejected unless the schema opts into values.
Generic abstraction (params / var / args). This waited until a real need appeared. The need was Change<Block>: an app plugs in its own block set and gets a strict Change through the whole op stack without copying it. Type variables pass through references (bind at the top, substitute everywhere) and fall back to a default when unbound, so non-generic use does not change. It stays self-describing: var is a new meta-schema variant, and params and args are maps of schemas on the existing variants. See generics and the schema language.
Open questions
Value constraints beyond kind. Since added. minLength/maxLength, pattern, minimum/maximum and minItems/maxItems came from the "Seed Blob Schema v1" dialect. They now narrow the values a leaf accepts. See the value-constraints section of the schema language. The meta-schema must be able to describe each one about itself. Each is cheap, but they add up, so the bar for more (string formats and so on) stays high: add one only for a real need.
Optional vs nullable. A missing key and a key set to null are different. Today required handles presence, and there is no nullable. This should be made explicit before data conventions settle.
Bytes constraints. Bytes are opaque. A future maxBytes, or a codec or media type tag, might matter for large or typed binary data. It is left out until needed.
Bundling recursive groups. Past question. The earlier design used a self-sentinel for direct self-reference and sketched "bundle into one block" for mutually recursive groups, without a bundle format. Named references replaced both: example/folder and example/file recurse through hm:// names with no bundle. See references.
Non-goals
Re-implementing JSON Schema. The language is intentionally tiny, and breadth is a non-goal.
A query or transformation language. It types data and nothing more.
Hiding IPLD. Links and content addressing stay visible on purpose.
See also
Why Hypermedia Schemas: the problem the design answers.
The schema language: every key these decisions produced.
The data model: the nine kinds.
References and naming: names, CIDs and the fixpoint problem.
Encoding: DAG-CBOR and the canonical form.
Blobs: content addressing on the Hypermedia Network.
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