# Forth

Implement an evaluator for a very simple subset of Forth.

[Forth](https://en.wikipedia.org/wiki/Forth_%28programming_language%29)
is a stack-based programming language. Implement a very basic evaluator
for a small subset of Forth.

Your evaluator has to support the following words:

- `+`, `-`, `*`, `/` (integer arithmetic)
- `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)

Your evaluator also has to support defining new words using the
customary syntax: `: word-name definition ;`.

To keep things simple the only data type you need to support is signed
integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a
sequence of one or more (ASCII) digits, a word is a sequence of one or
more letters, digits, symbols or punctuation that is not a number.
(Forth probably uses slightly different rules, but this is close
enough.)

Words are case-insensitive.

## Hints

To complete this exercise, you need to create the data type `ForthState`
and implement the following functions:

- `empty` returns an empty `ForthState`.
- `evalText` evaluates an input Text, returning the new state.
- `toList` returns the current stack as a list, with the element on top
of the stack being the rightmost (last) element.

You will find a dummy data declaration and type signatures already in place,
but it is up to you to define the functions and create a meaningful data type,
newtype or type synonym.



## Getting Started

For installation and learning resources, refer to the
[exercism help page](http://exercism.io/languages/haskell).

## Running the tests

To run the test suite, execute the following command:

```bash
stack test
```

#### If you get an error message like this...

```
No .cabal file found in directory
```

You are probably running an old stack version and need
to upgrade it.

#### Otherwise, if you get an error message like this...

```
No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...
```

Just do as it says and it will download and install
the correct compiler version:

```bash
stack setup
```

## Running *GHCi*

If you want to play with your solution in GHCi, just run the command:

```bash
stack ghci
```

## Feedback, Issues, Pull Requests

The [exercism/haskell](https://github.com/exercism/haskell) repository on
GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new
one, head over there and create an issue.  We'll do our best to help you!


## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.