S-Expressions
S-expressions use the same encodings as lists, but with different opcodes.
Opcode | Encoding |
---|---|
0xC0 -0xCF | Length-prefixed S-expression; low nibble of the opcode represents the byte-length. |
0xFB | Variable-length prefixed S-expression; a FlexUInt following the opcode represents the byte-length. |
0xF1 | Starts a delimited S-expression; 0xEF closes the most recently opened delimited container. |
0x5C | Tagless-element S-expression; opcode is followed by element encoding type and number of elements. |
0x8F 0x0B
represents null.sexp
.
Length-prefixed encoding
Length-prefixed encoding of an empty S-expression (()
)
┌──── An Opcode in the range 0xC0-0xCF indicates an S-expression.
│┌─── A low nibble of 0 indicates that the child values of this S-expression
││ took zero bytes to encode.
C0
Length-prefixed encoding of (1 2 3)
┌──── An Opcode in the range 0xC0-0xCF indicates an S-expression.
│┌─── A low nibble of 6 indicates that the child values of this S-expression
││ took six bytes to encode.
C6 61 01 61 02 61 03
└─┬─┘ └─┬─┘ └─┬─┘
1 2 3
Length-prefixed encoding of ("variable length sexp")
┌──── Opcode 0xFB indicates a variable-length sexp. A FlexUInt length follows.
│ ┌───── Length: FlexUInt 22
│ │ ┌────── Opcode 0xF8 indicates a variable-length string. A FlexUInt length follows.
│ │ │ ┌─────── Length: FlexUInt 20
│ │ │ │ v a r i a b l e l e n g t h s e x p
FB 2D F8 29 76 61 72 69 61 62 6C 65 20 6C 65 6E 67 74 68 20 73 65 78 70
└─────────────────────────────┬─────────────────────────────────┘
Nested string element
Delimited encoding
Delimited encoding of an empty S-expression (()
)
┌──── Opcode 0xF1 indicates a delimited S-expression
│ ┌─── Opcode 0xEF indicates the end of the most recently opened container
F1 EF
Delimited encoding of (1 2 3)
┌──── Opcode 0xF1 indicates a delimited S-expression
│ ┌─── Opcode 0xEF indicates the end of
│ │ the most recently opened container
F1 61 01 61 02 61 03 EF
└─┬─┘ └─┬─┘ └─┬─┘
1 2 3
Delimited encoding of (1 (2) 3)
┌──── Opcode 0xF1 indicates a delimited S-expression
│ ┌─── Opcode 0xF1 begins a nested delimited S-expression
│ │ ┌─── Opcode 0xEF closes the most recently
│ │ │ opened delimited container: the nested S-expression.
│ │ │ ┌─── Opcode 0xEF closes the most recently opened (and
│ │ │ │ still open)delimited container: the outer S-expression.
│ │ │ │
F1 61 01 F1 61 02 EF 61 03 EF
└─┬─┘ └─┬─┘ └─┬─┘
1 2 3
Tagless-Element S-Expressions
Opcode 0x5C
indicates a tagless-element S-expression. This is a compact encoding for homogeneous collections where all elements have the same type.
The elements of the S-expression can be a primitive encoding or a macro-shape.
The opcode is followed by:
- One or more bytes describing the tagless type:
- If the byte is in
0x00
-0x47
: only one byte (the opcode) is present; this is the macro address - If the byte is in
0x48
-0x4F
: it is followed by aFlexUInt
to encode the entire macro address - If the byte is
0xF4
: it is followed by aFlexUInt
which encodes the entire macro address - For any other byte value: only one byte (the opcode) is present
- If the byte is in
- A
FlexUInt
length indicating the number of direct child values in the S-expression - Each element encoded without the leading opcode or macro address
Tagless-element S-expression of integers (1 2 3 4)
┌──── Opcode 0x5C indicates a tagless-element S-expression
│ ┌─── Tagless type: 0x61 (int8)
│ │ ┌─── Length: FlexUInt 4 (4 elements)
│ │ │
5C 61 09 01 02 03 04
└────┬────┘
4 int8 values
Tagless-element S-expression with macro shape [\:point\ (1 3), (1 4), (2 4)]
┌──── Opcode 0x5C indicates a tagless-element sexp
│ ┌─── Tagless type: 0x05 (macro address 5, assuming :point is at address 5)
│ │ ┌─── Length: FlexUInt 3 (3 elements)
│ │ │ ┌─── First element: (1 3)
│ │ │ │ ┌─── Second element: (1 4)
│ │ │ │ │ ┌─── Third element: (2 4)
│ │ │ ┌────┴────┐ ┌────┴────┐ ┌────┴────┐
5C 05 07 61 01 61 03 61 01 61 04 61 02 61 04
└─┬─┘ └─┬─┘ └─┬─┘ └─┬─┘ └─┬─┘ └─┬─┘
1 3 1 4 2 4
Tagless-element S-expression with macro-shape using length-prefixed E-expression [\:3\ (1 3), (1 259) ]
┌──── Opcode 0x5C indicates a tagless-element sexp
│ ┌─── Tagless type opcode F4 with FlexUInt address 3
│ │ ┌─── Length: FlexUInt 2 (2 elements)
│ │ │ ┌─── First element: (1 3)
│ │ │ │ ┌─── Second element: (1 4)
│ ┌─┴─┐ │ ┌─────┴──────┐ ┌───────┴───────┐
5C F4 07 05 09 61 01 61 03 0B 61 01 62 04 01
│ └─┬─┘ └─┬─┘ │ └─┬─┘ └───┬──┘
│ 1 3 │ 1 259
└─ FlexUInt └─ FlexUInt
Length=4 Length=5
Encoding of null.sexp
┌──── Opcode 0x8F indicates a typed null; a byte follows specifying the type
│ ┌─── Null type: sexp
│ │
8F 0B