This page defines the schema-level vocabulary used by the reference.

A schema kind is a primary schema family. A schema group is a named set of schemas defined by a criterion. Exact field layouts, callable signatures, and other schema-specific details remain outside these broad categories.

• Schema Kinds
• Schema Groups
• Common operational consequences
• Example: schema kinds and groups
• What remains schema-specific
• Where to continue

Schema Kinds

Kind	Common sources	Description
Block	[...]	Compile-time block schema. Meta.
Code	codeRef, importFunction, exportFunction	Callable code schema whose exact signature belongs to the schema.
Cond	DEBUG, FALSE, TRUE, predicate results	Condition schema used by branching and logical condition operations.
Dict	{...} with named fields	Struct schema with named items.
Int8	Integer literals with i8	8-bit signed whole-number schema kind.
Int16	Integer literals with i16	16-bit signed whole-number schema kind.
Int32	Integer literals with default integer syntax or i32	32-bit signed whole-number schema kind.
Int64	Integer literals with i64	64-bit signed whole-number schema kind.
Intx	Integer literals with ix	Pointer-width signed whole-number schema kind.
List	array, uniform (...), uniform unnamed {...}	Struct schema with unnamed items of one shared item schema.
Nat8	Natural literals with n8	8-bit unsigned whole-number schema kind.
Nat16	Natural literals with n16	16-bit unsigned whole-number schema kind.
Nat32	Natural literals with n32	32-bit unsigned whole-number schema kind.
Nat64	Natural literals with n64	64-bit unsigned whole-number schema kind.
Natx	Natural literals with nx	Pointer-width unsigned whole-number schema kind.
Real32	Real literals with r32	32-bit floating-point schema kind.
Real64	Real literals with default real syntax or r64	64-bit floating-point schema kind.
Ref	@NAME, .@NAME, @, fieldRead	Reference schema kind. The schema defines the target schema and mutability.
Text	"TEXT", «TEXT», &	UTF-8 text schema kind.
Tuple	Non-uniform (...), empty (), non-uniform unnamed {...}	Struct schema with unnamed items whose item schemas are not required to match. The empty value () is a Tuple.

Schema Groups

A schema group is a named set of schemas defined by a criterion. Some groups are unions of schema kinds, while others are defined by broader schema properties.

Group	Criterion	Where it matters
Int	Schemas whose kind is Int8, Int16, Int32, Int64, or Intx.	Signed Int operations and comparisons.
Meta	Schemas of zero size.	Compile-time-only objects and schema-level operations. Tested by virtual?.
Nat	Schemas whose kind is Nat8, Nat16, Nat32, Nat64, or Natx.	Nat operations, counts, and address-like values.
Number	Schemas in the Whole or Real groups.	Arithmetic, numeric comparisons, casts, and numeric transforms.
Real	Schemas whose kind is Real32 or Real64.	Floating-point transforms such as cos, log, and sqrt.
Ref-like	Schemas whose kind is Ref, Code, or Text.	isRef and fieldIsRef.
Scalar	Schemas in the Cond kind or the Number group.	Scalar value operations and comparisons.
In-place	Schemas whose values are stored directly at their own runtime location.	Default runtime data path. Numbers, Cond, and non-Meta Struct schemas are In-place. Contained fields may still be Ref-like; the term describes the outer schema, not recursive field contents.
Struct	Schemas whose kind is Dict, List, or Tuple.	Struct schema operations such as isCombined, fieldCount, fieldIndex, fieldName, and has.
Whole	Schemas in the Int or Nat groups.	Bitwise operators and shifts such as and, lshift, rshift, xor, and ~.

Common operational consequences

The groups above are schema-level sets. The recurring consequences below summarize common implications of those groups.

• In-place locals and non-static In-place fields use the default runtime-object path. Mentioning them yields an immutable Ref-like result, reading them yields a Ref result, and writing to them replaces the stored value itself.
• Ref-like values use special value rules of their own kinds. Ref copies or replaces references, Text copies text values, and Code follows callable-code rules.
• Meta values are compile-time objects. Operations that require runtime object storage reject them or create fresh compile-time objects instead of reading or writing stored runtime objects.
• Struct is the union of Dict, List, and Tuple, so schema operations such as fieldCount, fieldName, and isCombined are stated through that group.
• Whole, Number, Scalar, Int, Nat, and Real are arithmetic grouping terms used to state builtin acceptance and result rules concisely.

Example

Schema kinds and groups

{} 0 {} [
  "-- schema kinds --" printCompilerMessage
  [] schemaName printStack _:;
  {x: 0;} schemaName printStack _:;
  (1 2) schemaName printStack _:;
  0n32 schemaName printStack _:;
  "hi" schemaName printStack _:;

  "-- schema groups --" printCompilerMessage
  {x: 0;} isCombined printStack _:;
  (1 2) isCombined printStack _:;
  [] code? printStack _:;
  {} () {} codeRef codeRef? printStack _:;
  () virtual? printStack _:;
  0n32 managed? printStack _:;

  0
] "main" exportFunction

Expected Output During Compilation

-- schema kinds --
"«test.mpl» 3 3 Block"
"{x: Int32;}"
"Int32 2 array"
"Nat32"
"Text"
-- schema groups --
TRUE
TRUE
TRUE
TRUE
TRUE
FALSE

What remains schema-specific

• Mutability is not a schema kind or schema group.
• Known versus unknown is not a schema kind or schema group.
• Stable versus dynamic is not a schema kind or schema group.
• Static declaration state is not a schema kind or schema group.
• Exact callable signatures are not schema kinds or schema groups.
• Exact Dict, List, and Tuple item layouts are not schema kinds or schema groups.

Where to continue

• For syntax that creates these schemas, see Node Reference.
• For operations that inspect or manipulate them, see Built-in Reference.
• For exact schema predicates, see builtins such as code?, codeRef?, isCombined, managed?, same, and virtual?.