msgpack/encode

(Cons A) -- the decoded elements, or the empty list.

(:: (decode-mp-list t (mp-tree-root t)) (Cons int))

(defstruct User [id : int  name : cstr  active : bool])
  (derive-msgpack User (id int) (name cstr) (active bool))
  ;; (encode-mp (make-struct User 7 "alice" true))
  ;;   =>  83 a2 69 64 07 a4 6e 61 6d 65 a5 61 6c 69 63 65 a6 61 63 74 69 76 65 c3

(defstruct LogEntry [level : cstr  msg : cstr])
  (derive-msgpack-encode LogEntry (level cstr) (msg cstr))

(defstruct Config [port : int  host : cstr])
  (derive-msgpack-decode Config (port int) (host cstr))
  ;; (:: (decode-mp t root) (Result Config cstr))

(defopaque UserId :int)
  (derive-msgpack-opaque UserId :as int)
  ;; (encode-mp (:: 42 UserId))                    => 0x2a
  ;; (:: (decode-mp t n) (Result UserId cstr))     => parses an int as UserId

(defdata Event :copy [] (Click :int :int) (Scroll :int))
  (derive-msgpack-sum Event (Click (x int) (y int)) (Scroll (dy int)))
  ;; (encode-mp (Click 3 4))  => 81 a5 43 6c 69 63 6b 82 a1 78 03 a1 79 04

Limitations (carried from the struct slice):
  - Field-name policy is identity.
  - Inner field decode does not accumulate structured errors.
  - A nullary constructor encodes as `{"Ctor":{}}` (an empty inner map).

int -- the violation count.

(mp-decode-errors-count (err-val r))   ; => 2

cstr -- a borrowed key name (the empty string when i is out of range).

(mp-decode-error-path errs 0)   ; => "age"

cstr -- a borrowed schema type name ("int", "string", ...).

(mp-decode-error-expected errs 0)   ; => "int"

cstr -- a borrowed schema type name, or "missing" when the key was absent.

(mp-decode-error-got errs 0)   ; => "string"

(mp-decode-errors-free (err-val r))

(defstruct Person [name : cstr  age : int])
  (derive-mp-decoder Person (name cstr) (age int))
  ;; (:: (decode-mp-checked t root) (Result Person MpDecodeErrors))
  ;;   {"name":"al","age":30}  => (ok <Person>)
  ;;   {"name":123}            => (err <2: name/string/int, age/int/missing>)

Limitations (v0): primitive fields only (nested maps and arrays are a
follow-up), the path is the bare key with no dot-nesting, and the field cap
follows `__mp-decode-make-struct` (<=5). For a richer shape, hand-write the
DecodeMpChecked instance.