#+title: representation of Scheme types

the Scheme unitype is encoded as ~(ref eq)~ with immediates in ~(ref i31)~ and heap objects in ~$heap-object~:
#+begin_src wat
  (type $heap-object (sub (struct (field $hash (mut i32)))))
#+end_src

* immediates

all immediates are stored in ~(ref i31)~ and thus must fit in 31 bits. the most important immediate, the integer, is indicated by a null low bit.
#+begin_example
XXXX XXXX XXXX XXXX XXXX XXXX XXXX XX00
                                     || 
                                     |\ used by wasm's i31 rep
      zero indicates a 30-bit fixnum /
      in the upper bits
#+end_example

| type/value | low bits |
|------------+----------|
| small int  |        0 |
| ~false~      |       01 |
| ~true~       |       11 |