doc: add developer guide

readme.md

@@ -86,6 +86,10 @@ yorl sample/test.yl
 
 You should get something like: ``báwo ni ayé``
 
+## Developer Guide
+
+For Developers, [these docs](./src/README.md) will help you understand Yorlang's internal architecture, should you wish to make adjustments.
+
 ## Unit and Integration Tests
 
 To run tests in the project, run the following in your terminal:

src/README.md

@@ -0,0 +1,63 @@
+# Developer Guide
+
+Yorlang is an interpreted programming language built on NodeJS.
+
+When you run `yorl <file>.yl` in your terminal, the `yorl` command is handled by the [cli.js](../cli.js) file, which uses [commander](https://www.npmjs.com/package/commander) to start the Yorlang interpreter, which performs the following actions:
+
+## InputStream
+
+The file path is passed into the [InputStream](./inputstream.js) instance, which reads the file's contents as a string.
+
+An InputStream contains methods such as `next`, `peek` and `isEndOfFile`.
+
+### - next()
+
+It maintains a cursor position which begins at the beginning of the file, and everytime `next()` is called, the character immediately after the cursor is returned, and the cursor moved one-char forward.
+
+> This [repl](https://repl.it/@mykeels/yl-input-stream-next-demo) shows how InputStream's `next()` works.
+
+### - peek()
+
+Rather than update the cursor position, the `peek()` function returns the character immediately after the cursor, and keeps its position the same.
+
+### - isEndOfFile()
+
+This returns a boolean value when the cursor has reached the end of the input file.
+
+> This [repl](https://repl.it/@mykeels/yl-input-stream-peek-demo) shows how InputStream's `peek()` and `isEndOfFile()` work.
+
+## Lexical Analyser
+
+The [Lexer](./lexer.js) instance accepts an InputStream as an argument, and attempts to read each character, and recognise token such as identifiers, operators, keywords, and numbers.
+
+A Lexer contains methods such as:
+
+### - readWhile(predicate)
+
+This will retrieve the next characters from the InputStream until a character is found that does not match the condition given by a predicate function.
+
+> `readWhile((c) => c != ";")` will read characters until either one of them matches `";"`, or the end of the file is reached.
+
+### - readNext()
+
+This makes sure to skip whitespaces and comments, and depending on the nature of a character read, it calls and returns other methods such as:
+
+### - readString()
+
+This is called by `readNext()` when a quote character is found. It will attempt to read the characters until a closing quote is found.
+
+### - readNumber()
+
+This is called by `readNext()` when a digit is found. It will attempt to read a valid number (including dots), until no more digits can be found.
+
+### - readIdentifier()
+
+This is called by `readNext()` when a character is found that matches the `constants.REGEX.IDENTIFIER` regex pattern. It will attempt to read subsequent characters that match the pattern, till either one is found that does not, or the end of file is reached.
+
+### - isPunctuation()
+
+This determines whether a character read is a punctuation. The list of punctuations can be found in `constants.LIST.PUNCTUATIONS`.
+
+### - isOperator()
+
+This determines whether a character read is an operator. The list of operators can be found in `constants.LIST.OPERATORS`.