Slicer is an interpreter for a simple ML-like language called iTML (imperative Transparent ML). This is companion code for paper "Imperative Functional Programs that Explain their Work" by Wilmer Ricciotti, Jan Stolarek, Roly Perera and James Cheney, published at International Conference on Functional Programming 2017. Upstream source code repository is located at https://github.com/jstolarek/slicer.
This software is written in Haskell. To build and install it you will need GHC 8.0 or newer and a typical Haskell infrastructure (Cabal, access to Hackage).
Get the sources:
git clone https://github.com/jstolarek/slicer cd slicer
If you want to work on the exact version that was submitted for the ICFP 2017 artefact evaluation check out the relevant tag:
git checkout icfp2017-final
Initialize cabal sandbox:
cd slicer cabal sandbox init
cabal install --dependencies-only
If you want to run tests or benchmarks add
--enable-benchmarksrespectively to the above command.
Configure and build the project
cabal configure cabal build
After following the above steps the slicer executable will be located at
dist/build/slicer/slicer. Example TML programs are located in
sub-directory. You can run them by passing file name on the command line:
$ ./dist/build/slicer/slicer examples/icfp17-example.tml
If all went well you should see the output:
Running examples/icfp17-example.tml val it = let f = (fun f x => if x == 0 then y := 6 * !z else _) in try f 0 with _ => _ ;; !y : trace(int)
You can pass more than one file name at the same time.
Slicer provides a very simple REPL. Invoke it by passing
$ ./dist/build/slicer/slicer --repl Welcome to Slicer REPL slicer>
You can interact with REPL by providing single-line declarations and expressions:
slicer> data intlist = Nil | Cons int * intlist slicer> let tail = fun tail (xs : intlist) : intlist => case xs of Nil -> Nil; Cons xs' -> snd xs' val it = (fun tail xs => case xs of Nil -> Nil; Cons xs' -> snd xs') : (intlist -> intlist) slicer> let t = trace (tail (Cons (1, Cons (2, Cons (3, Nil))))) val it = trace (tail Cons (1, Cons (2, Cons (3, Nil)))) : trace(intlist) slicer> bwdSlice (t, Cons (2, _:intlist)) val it = tail Cons (_, Cons (2, _)) : trace(intlist)
Running the test suite
All example programs are packaged into a single test suite. Build and run the testsuite with:
cabal install --dependencies-only --enable-tests cabal build cabal test
The test suite will run every program in
examples directory and compare its
output with the expected output stored in a corresponding
*.golden file inside
Reproducing examples from the paper
In the paper we have discussed several examples. These are all located as
source files inside
On the first page of the paper we present a following example:
let f(x) = if (x == 0) then y := 6 * !z else (y := 84 / !z; w := g(!y + 12)) in try f 0 with x => y := 42);; !y
This example can be found in file
icfp17-example.tml. Run it with:
$ ./dist/build/slicer/slicer examples/icfp17-example.tml
You can tweak initial value of
zand the value of argument passed to
fto obtain different results. Remember that you must change the slicing criterion to match the obtained result. So for example if you change initial value of
7you need to change the slicing criterion from
0. If you remove the
withexception handler and modify the code to cause division by zero the slicing criterion should be
raise "Division by zero".
On the third page of the paper we present a more elaborate example containing a usage of
map. This example can be found in
icfp17-example2.tmlsource file. Run it with:
$ ./dist/build/slicer/slicer examples/icfp17-example2.tml
In section 6 we discuss an iTML program that solves a system of linear equations by using simple Gaussian elimination without pivoting. This program can be found in
gauss.tmlsource file (its pretty-printed version can be found in Appendix C in the extended version of the paper). Run it with:
$ ./dist/build/slicer/slicer examples/gauss.tml
In this program we define two systems of equations. System defined by arrays
bsis solved correctly. System defined by arrays
bs'causes a division by zero. We then map
gaussfunction over a list containing arrays defining both systems. This allows us to demonstrate that our method works in a higher-order setting even for complex functions. Note that we slice the program w.r.t. "Division by zero" exception. That is because we know our program divides by zero and we want to understand why. After running the program observe the results:
all elements of arrays
bsare replaced with holes. This means that neither of them contributed to raising the exception.
some elements of
as'are replaced with holes and some are not. This means that some elements of that array contributed to raising the exception. The last of the elements that was not replaced with a hole is located at row 2, column 1 (0-based indexing). Knowing the nature of Gaussian elimination leads us to conclusion that this is where the division by zero must have taken place.
in the definition of
gaussmethod code responsible for zeroing elements above the diagonal has been replaced by a hole. This means that this code did not contribute to the result and so the exception was raised when zeroing elements below the diagonal.
Syntax of iTML is based on ML. Here are some basic information that should get you started:
built-in primitive types are:
iTML does not support polymorphic types
you can define your own ADTs but iTML only allows them to have two constructors. Moreover, constructors can only be nullary or unary. So a list of integers would be defined as:
data intlist = Nil | Cons (int * intlist)
mapfunction defined over a list of integers (no polymorphism - remember!) looks like this:
let map = fun map (f : int -> int) (xs : intlist) : intlist => case xs of Nil -> Nil; Cons y -> Cons (f (fst y), map f (snd y))
Note the calls to
snd- these are a consequence of storing head and tail of a list inside a tuple (
Conscan be at most unary).
all data declarations should be placed before the program code
iTML does not support top-level bindings at the moment. In other words, once you define data types the remaining program is just a single expression.
parser currently binds operators tighter than function application. So for example
fst (raise "foo") + 2is parsed as
fst ((raise "foo") + 2)rather than
(fst (raise "foo")) + 2. Use parentheses to disambiguate.
enclose code inside a
traceprimitive to trace that code. Use
bwdSlicefor backward slicing:
let t = trace ( (* YOUR CODE HERE *) ) in bwdSlice (t, (* SLICING CRITERION HERE *) )
With all that in mind we recommend that you look at programs in
Source code tour
Source code is divided into a library and an executable that depends on the
library. The library is located in
lib directory. Under
lib/Language/Slicer/ you will find these modules:
Absyn: abstract syntax for TML language. This is what the parser produces (
Core: abstract syntax tree for the core language. Produced from TML abstract syntax tree (
Absynmodule) during desugaring (
Desugarmodule). Contains also built-in operators and functions for working with the store abstraction.
Desugar- translates TML abstract syntax tree (
Absynmodule) into core language (
Env- definitions of variable and type environments + helper functions.
Error- data type for handling compilation errors.
Eval- evaluation of core programs
Monad/sub-directory - monads for running various stages of the interpreter pipeline.
Parser- well, the parser.
Primitives- built-in language primitives
Resugar- translate core programs into surface-like syntax. Used for pretty-printing.
Run- entry points to the interpreter
Slice- implementation of slicing
UpperSemiLattice- lattice definition
Executable is located inside
Main- executable logic (parsing command line options, invoking the library, etc.)
Monad/- REPL monad
Repl- REPL logic
This is experimental research software. It comes as-is without any guarantees.