(A Time Travelling Interpreter)

Delorean is a toy interpreter for a toy statement language that supports stepping forward and back through a program's execution, with various inspection tools. It also does rudimentary static analysis.

The wonderful Docopt is used to define and parse the CLI. To run on a particular program:

$ stack build
$ stack exec delorean-exe -- <infile.txt>

Delorean will launch into the debug prompt. The current statement to be executed can be displayed with the list command (or, as with all of the commands, its initial l)

___  __ \_______  /________________________ _______
__  / / /  _ \_  / _  __ \_  ___/  _ \  __ `/_  __ \
_  /_/ //  __/  /__/ /_/ /  /   /  __/ /_/ /_  / / /
/_____/ \___//_____|____//_/    \___/\__,_/ /_/ /_/
The Amazing Time-Travelling Interpreter!
type `help` or `h` for a list of commands

delorean> l
a <- 50: Int
b <- 10: Int
b <- 30: Int
c <- a + b
print c
if b > a then
  print b
else
  print a
fi
delorean>

There is some amount of show trickery to get basic pretty printing of source listings.

To execute lines of the program, use step, or s for short:

delorean> l
a <- 50: Int
b <- 10: Int
b <- 30: Int
c <- a + b
print c
if b > a then
  print b
else
  print a
fi
delorean> step
delorean> s
delorean> i a
50: Int
delorean>

The value of a particular variable can be inspected with the inspect <varname> command. This will print out a list of all of the values which <varname> has taken on up until the current line of execution, most recent at the top.

...
delorean> i a
50: Int
delorean> i b
30: Int
10: Int
delorean>

As well as inspection of a single variable, changes shows all of the different states that all of the variables have been in up until the current point:

delorean> c
a -> 50: Int, b -> 30: Int   -- `a` is of type Int and mapped to the value 50
a -> 50: Int, b -> 10: Int
a -> 50: Int

As well as being able to step forward, Delorean can also step back through the history of the program. At each execution of a statement, the state of the program along with that statement is pushed onto the 'life' stack.

When stepping back, a (Statement, Environment) pair is popped off, and the Statment Prompted for execution, the effect being that step now executes the last-popped statement, before executing the statement the program was on before back was issued:

delorean> list
a <- 50: Int
delorean> step
delorean> list
b <- 10: Int
delorean> step
delorean> l
b <- 30: Int
delorean> back
delorean> l
b <- 10: Int
delorean> b
delorean> l
a <- 50: Int

Carefull not to step back to before the start of the program; in that case the interpreter will, unfortunately, crash.

dump will dump out the (Statement, Environment) stack used when stepping back. This is mainly useful for debugging the interpreter itself, but sure whats the harm in leaving it in:

delorean> d
[(b <- 30: Int,fromList [("a",50: Int),("b",30: Int)]),(b <- 10: Int,
fromList [("a",50: Int),("b",10: Int)]),(a <- 50: Int,fromList [("a",
50: Int)]),(pass,fromList [])]

help lists all available commands:

h help    : Print this message
s step    : Execute one statement of the program
b back    : Undo one statement of the program
l list    : List the current statement
c changes : List all different states of the program
d dump    : (debug) Dump out all runtime data
i inspect <variable name>:
     Inspect given variable's content

Originally the plan was to use GADTs to grant static types to the Expression language, but that ended up being a lot more troublesome than initially expected.

What's much easier to do is to check for declared, but unused variables!

The simple approach is that {unused variables} = {declared variables} \ {used variables}. We can easily define a declared variable as any variable appearing in an (Assign <name> <expr>) statement, and a used variable as any variable appearing in any Expression anywhere in the program.

Upon opening delorean on a program with unused variables, a basic message is printed out:

___  __ \_______  /________________________ _______
__  / / /  _ \_  / _  __ \_  ___/  _ \  __ `/_  __ \
_  /_/ //  __/  /__/ /_/ /  /   /  __/ /_/ /_  / / /
/_____/ \___//_____|____//_/    \___/\__,_/ /_/ /_/
The Amazing Time-Travelling Interpreter!
type `help` or `h` for a list of commands

Yikes, variable b is defined but never used!
delorean>

Or for a larger example:

Great Scott! The variables b, c, d, e are defined, but never used!
delorean> list
a <- 50: Int
b <- 10: Int
c <- 10: Int
d <- 10: Int
e <- 10: Int
print a