Bind & Interpretation of AppC in AAQZ 4

1. Bind

adding new expressoion

 <expr>::= ...
        | {bind <clause> ... <expr>}

<clause>::= [<id> = <expr>]

1 {bind [x = 17] {* 2 x}}

2 {bind [add 1 = {(x) => {+ x 1}}] …}

if we already have the langauge that supports this feature we can just make it parse into the exisiting abstract syntax.

WE DONT NEED A BINDC

So all we have to do is add a clause to the parser…

{bind [name = val] body} --> {{(name) => body} val}

Bind is just a AppC & LamC.

Bind is an example of syntatic sugar.

Syntatic Sugar: syntax that you may prefer but the rest of the PL needs to know nothing about.

Desugaring: moving the sugared code into stuff the program has and can use.

Example:

{bind [x=5] [x=7] {+ x y}} -desugar-> {{(x y) => {+ x y}} 5 7} -parse-> (AppC (LamC ’(x y) (AppC (IdC ’+) (list (IdC ’x) (IdC ’y))))….

common mistakes

once the first clov is interpreted naming the match for the clov the same as current env
Extending the environment to the wrong CloV

{bind [f = {(x) => {(x) => {+ x y}}}] {{f 3} 7}}

func = {f 3}

arg = 7

Interp func in the current envionment
- CloV
  - Param : Y
  - body : {x + y}
  - env : x -> (NumV 3)
  - stuff always in env: + - * / true false
Interp the the arg interpret arg in current environment
Extend the closures env with new bindings from closures params to interpet args.
- Extension of the previous environment with new things
  - y -> 7, x ->> 3, + -> …
Interp the closure body with the new extended environment
- books approach uses cons to extend the enviornments on the top