# Implementing a small imperative language
A video on this section can be found [here](https://bham.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=c37094a4-9a88-4803-8f73-ac85012eb81b).
We implement an [interpreter](https://en.wikipedia.org/wiki/Interpreter_(computing)) for a small imperative programming language.
## Example
This is the sample file [fibonacci.xy](fibonacci.xy):
“`java
{
y := 0;
z := 1;
while (x > 0)
{
x := x – 1;
t := y + z;
y := z;
z := t;
}
}
“`
There are no IO commands in our language. The input is in the variable `x` and the output is in the variable `y`. Hence we call our language `xy`. We run this as follows:
“`
$ runhaskell Runxy.hs fibonacci.xy 11
89
“`
## Overall structure
Given a program source code, that is, a `String`, in *concrete syntax* as above:
* We parse it, to produce a program tree in [abstract syntax](https://en.wikipedia.org/wiki/Abstract_syntax_tree).
* Given this and an initial storage, that is, an assignment of values to program variables, we run the program to get a new storage.
“`
| initial
| storage
v final
source code +———–+ program tree +——-+——–+ storage
———————>| Parser.hs | ——————–>| Interpreter.hs | ——–>
(concrete syntax) +———–+ (AbstractSyntax.hs) +—————-+
(a string)
“`
We use a program [Runxy.hs](Runxy.md) to read a file into a `String` and read a value `x` from the command line arguments, which then calls the parser and then the interpreter, with an initial storage assigning the value `x` to the program variable `”x”` and finally prints the value of the variable `”y”`, as in the above example.
#### Next: [Concrete syntax](ConcreteSyntax.md)