**MatRepl**
* is a Matrix
* and a repl: Read–Eval–Print Loop, where Print is doing operations on vectors and matrices in a graphic environment
* written in vanilla javascript (ES6)
* see live at https://matrepl.net

![add 2 vectors](screenshot.png? "add 2 vectors")
  
The repl has the following syntax (It's work in progress, new capabilities will be added)
* arithmetic expressions:
  * add(+), subtract(-), divide(/), multiply(*) on scalars, vectors and matrices. 
* variable declaration eg: ```a = 1 + 2```
* ```remove(x)``` removes bindings (when it's an object (eg vector), removes it from the matrix)
* ```remove(@n)``` removes an object using it's assigned index (```n``` is a number)
  * By the way, values can be bound to a name (assigned to a variable), but you can always also refer to them using their id, using : ```@n```
  * so ```a = vector(1,1)```
  > > vector@0{x0:0, y0: 1, x:1, y:1}
  * and then ```@0```
  > > vector@0{x0:0, y0: 1, x:1, y:1}
* method calls:
   ```a = vector(12, 1)``` works as well. The start is now the origin.
  > > vector@0{x0:0, y0: 2, x:12, y:1}
  
  ```vector(-1 -1)``` != ```vector(-1-1)``` and the latter would mean ```vector(-2)``` which is not legal.
* arrays: \[i<sub>0</sub>, i<sub>1</sub>, i<sub>2</sub>, ... i<sub>n</sub>] creates an array. 
  * \[i<sub>0</sub>, i<sub>1</sub>] is special: it is a shorthand for creating a 2-dimensional vector. I'll have to think of another way to create an array of length 2...
 
* ```id()`` adds a 2d identity matrix, which is also the two basis vectors.  
* properties
  * ```a = [1,2]```
  * ```a.type```
  > &gt; vector
  * ```a.x+1```
  > &gt; 13
* drag vectors using the mouse pointer. You can change the vector arrows visually
* lazy evaluation. The difference between ```c = a+b``` and ```c = "a+b"``` 
  is that the latter assigns to ```c``` a parsed expression that can always be evaluated later. 
  When you apply lazy evaluation and later update ```a```, the value for c will be reevaluated
  automatically. Combined with vector dragging, this way you can get an intuition for vector addition.
  Want to do the same for matrix multiplication and basis change. 

* Example: enter the following, like in the screenshot:
  ```
  a = vector(0.5, 0.5)
  b = vector(-1, 1)
  c = "a + b"
  ```
* and press enter. Then using the mouse pointer move vector ```a``` or ```b```. Or try: ```a = 2 * a```
  This updates vector ```a``` to twice it's size. And, because ```c``` is defined lazily, it is updated as well!


**To run locally**
* make sure you have node/npm
* cmdline: npm run start
* and enter ```help()```