Libsexp

An S-Expression parser library, written in Nim.

On usage

There are two important functions, decode and encode.

proc decode*(input: string): Exp

Decode takes an input string and outputs an S-Expression object. If any error occurs, it returns an error.

proc encode*(input: Exp): string

Encode takes an S-Expression object and returns a string that contains the S-Expression representation of that object.

On type hierarchy

An S-Expression can be one of several types.

List types:

• List (), (+ 1 2 3), (+ 2 (* 3 4))

Value types:

• Nil
• String "Hello, world!"
• Int 42, -69
• Float 4.20
• Keywords #foobar, #error
• Boolean (subtype of keyword) #true or #false
• Symbol +, foobar, write-file

The order in which there are listed, is the order in which they are parsed. Thus there is a hierarchy of types, which determines the output of the decode function.

On keywords

Keywords are different from symbols, in that keywords won't evaluate to anything. The symbol foo may evaluate to a procedure, a number, or even an keyword. The keyword #foo will always resolve to #foo.

There are two special keywords:

• Boolean true #true
• Boolean false #false

On cons

type Cons* = tuple[car: Exp, cdr: List]

Cons or construct cells are the abstraction used to deconstruct a list to its component parts. Namely car and cdr, which I use to get the procedure and operand symbols. For convenience there are conversion functions.

proc toCons*(list: List): Cons

This function takes in a list and returns a construct cell.

proc toList*(cons: Cons): List

While this functions takes in a construct cell, and return its combination as a list.