Logic Programming in F#

Code and Examples from John Harrison's "Handbook of Practical Logic and Automated Reasoning"

Purpose

Handbook of Practical Logic and Automated Reasoning is a book designed to teach the fundamental aspects of propositional logic, automated theorem proving, and proof assistants. It includes a large number of examples written in OCaml, which we have translated and adapted to F# in order to take advantage of Visual Studio and the .NET Framework.

Our ported F# code aims to stay as close as possible to the original OCaml to make it easy to follow along with the book.

Setup / Installation

There are two solutions: *.VS10.sln and *.VS11.sln for Visual Studio 2010 and Visual Studio 2012, respectively. Both solutions target .NET 4.0.

NuGet is used to manage external packages; the easiest way to install NuGet is by downloading it (for free) from the Visual Studio Extension Gallery. If you do not have NuGet, or are running a version prior to 2.5, you must install it (or upgrade) before you will be able to build the projects.

The solution uses the Package Restore feature of NuGet to automatically download any missing packages when the project is built. This requires that you have the "Allow NuGet to download missing packages during build" setting enabled; in Visual Studio, you can find the setting under Options -> Package Manager -> General.

Once NuGet is installed and configured, you should be able to build the solution.

See Install NuGet in the wiki pages for step by step instructions.

When reading along with the book

The OCaml code from resource page combines the code and examples in one script. We have separated them in our ported F# code to simplify development and testing; all library code has been placed into the FSharpx.Books.AutomatedReasoning project, and the examples put into F# scripts (.fsx) in the Examples folder.

In the book, OCaml code and example scripts are identified by bounding boxes. Example code typically starts with an #, indicating it is being run in the OCaml REPL; the ported code for these are typically found in the Examples directory and code without the # is typically found in the library project.

In the OCaml code, the examples are demarked by START_INTERACTIVE and END_INTERACTIVE.

Running Examples

To run the example scripts, you must first build the solution (see the previous section).

The Examples folder contains *.fsx scripts corresponding to the original OCaml files. Within each *.fsx file, examples appear in the same order as in the book; we have added a comment to each example with its page number in the book to make cross-referencing easier.

When first opening an example script file, run the #load, open and fsi.AddPrinter statements at the top of the script file to setup the interactive environment for the following examples.

We suggest that when running the examples, you run each one separately so as not to lose track of which example produced which result. Some of the examples rely on statements earlier in the script, so if you skip ahead you may get errors.

In order to demonstrate certain aspects and limitations of automated reasoning, some of the examples purposely fail by raising an exception; others may take several minutes, hours, or days to run -- if they terminate at all. In both cases, these examples are identified by comments in the scripts so you will know this is the expected behavior.

Each of the examples fall into one of the following categories (with the associated special comment, if applicable):

Completes successfully in <10 seconds.

No special comment. Most examples fall into this category.

Completes successfully in >5 minutes.

Includes a comment with the result of the fsi #time directive when the example is run on an average machine. For example:

// Real: 00:04:35.586, CPU: 00:04:31.718, GC gen0: 5020, gen1: 137, gen2: 9

 Note: F# Interactive #time directive either truncates or wraps values when the time is > 24 hours.

Runs for an unknown or infinite length of time

Some examples take a very long time to run. These are marked with the comment LongRunning.

Failure

Marked with a comment noting the expected reason for failure.

Exception

Examples that raise a non-Failure exception; marked with a comment noting the expected type of the exception.

Notable differences between the OCaml and F# code

In a few places, errors are handled using the Option type instead of exceptions. This is because:
- Exceptions in F#/.NET are much slower than in OCaml
- Recursive functions which throw and/or catch exceptions don't play nicely with type inference (in both F# and OCaml); the output type of such functions cannot be unified, so it will always be shown as a generic parameter (e.g., 'b) instead of the true output type of the function (e.g., int or bool). These functions also make it extremely difficult to locate the source of an error.
Preprocessors such as camlp4 / camlp5 don't exist for F#. The original OCaml code for the book uses camlp5 quotations (<< and >>) to transparently call the parsing functions for formula and term strings. Our F# code adds explicit calls to the parsing functions; as such, no uses of default_parser and default_printer appear in the F# code.

Examples:
```
(* OCaml *)
<<x + 3 * y>>;;
```
```
// F#
parse_exp "x + 3 * y";;
```
```
(* OCaml *)
<<p ==> q <=> r /\ s \/ (t <=> ~ ~u /\ v)>>;;
```
```
// F#
parse_prop_formula "p ==> q <=> r /\ s \/ (t <=> ~ ~u /\ v)";;
```
Duplicate names are avoided. Since OCaml shadows names and F# does not allow duplicate names, any function name causing a duplicate name error will have the name appended with an increasing sequential number.
```
(* OCaml *)
let a = ...;;
let a = ...;;
let a = ...;;
```
```
// F#
let a001 = ...;;
let a002 = ...;;
let a003 = ...;;
```
For some of the test strings such as in tableaux.fsx, the same name is used multiple times. To avoid duplicate name errors some of the names have a character appended.
```
(* OCaml *)
let p20 = prawitx ...;;
let p20 = compare ...;;
let p20 = splittab ...;;
```
```
// F#
let p20p = prawitx ...;;
let p20c = compare ...;;
let p20s = splittab ...;;
```
Some OCaml toplevel commands such as #trace and #install-printer don't exist in F# Interactive (fsi). In some cases, the functionality can be replicated:
```
(* OCaml toplevel *)
#install_printer my_printer;;
```
```
// F# (fsi)
fsi.AddPrinter my_printer;;   // my_printer : 'T -> string
```

Unit Testing

The FSharpx.Books.AutomatedReasoning.Tests project contains all of the examples from the book (plus some additional examples from John Harrison's website), converted into unit test cases with NUnit 2.6.3 and FsUnit 1.1.1.0. These test cases serve as evidence (but not proof!) of correctness as the code base is updated or optimized over time.

You can execute the tests by building the FSharpx.Books.AutomatedReasoning.Tests project, then loading the compiled assembly into a test runner like NUnit GUI or TestDriven.NET 3.4.2808 (RTM).

We strongly recommend using the x86 versions of the test runners. The CLR's default maximum stack size of 1MB is enough for 32-bit processes, but the test cases reliably crash with a StackOverflowException on a 64-bit process. This is because many of the library functions are recursive, but not tail-recursive -- and since many types double in size on an x64 platform, these functions quickly consume the stack and crash the process.

NOTE: On a 64-bit machine, the NUnit installer only creates a Start Menu shortcut for the 64-bit version so you must create your own shortcut (e.g., on the Desktop) to the x86 version; the x86 version is normally found at C:\Program Files (x86)\NUnit 2.6.3\bin\nunit-x86.exe.

Once you have installed NUnit, built the FSharpx.Books.AutomatedReasoning.Tests project, and opened the NUnit (x86) GUI, follow these steps to run the tests:

In the NUnit GUI, go to File -> Open Project. Find the FSharpx.Books.AutomatedReasoning.Tests.dll in the FSharpx.Books.AutomatedReasoning.Tests\bin\Debug folder in your repository folder (i.e., the folder you cloned the repository into). Double-click the file, or select it and press the 'Open' button.
Click the Categories tab on left side of the NUnit GUI window.
Double-click 'LongRunning', then check the box labeled Exclude these categories.
Click the Tests tab on left side of the NUnit GUI window and press the 'Run' button.

There are a few important points to note when implementing new test cases:

NUnit only accepts parameterized tests on primitive types. To compare sophisticated values, we have to put them into arrays and use indices as test parameters.
FsUnit uses type test to implement its DSL. Type inference doesn't work on this DSL, so make sure that two compared values belong to the same type.
FsUnit and the library have some clashed constraints, namely True and False. To create tests correctly, one might need to use detailed type annotation such as formula<fol>.True and formula<fol>.False for literals in first-order logic.
A few slow tests are put into LongRunning category. These tests aren't recommended for normal development -- they're only used to validate the ported code on release.

F#-specific porting notes

The F# compiler warning FS0025 (about incomplete pattern matches) has been disabled for the Debug configurations of the projects. These incomplete matches are found throughout the original OCaml code and eliminating them correctly would require extensive changes to the code. So, to keep our code faithful to the book, it has been left as-is and the warning disabled to promote readability.
A few examples must be run with a 16MB stack (the default limit set by OCaml version); this is done using the runWithEnlargedStack function in Examples\initialization.fsx. Without using our runWithEnlargedStack function, these examples crash with a StackOverflowException because the CLR uses a 1MB stack by default. (Discussion: Why does F# impose a low limit on stack size?).
The built-in Failure active pattern has been redefined to accommodate KeyNotFoundException, ArgumentException, etc. The OCaml version makes use of Failure as a control flow; the F# version throws different kinds of exceptions which aren't caught by the default Failure pattern. The active pattern may need to be updated to handle other exceptions later (see the detailed function in the beginning of lib.fs).

Traceability

The primary method for checking the F# code was to compare the results of the OCaml output against the F# output. There are several hundred examples that need to be compared and doing so without a means of tracking is tedious at best. To make it easier to trace from an F# example's output back to the OCaml example's output, each example was given a unique identifier, which is the combination of the module name with a sequential number. e.g. complex.p001. These identifiers are referred to as traceability comments.

The traceability comments were put into a version of the OCaml source code, a run of the OCaml code, the F# examples and the F# unit test. Currently the OCaml code with the traceability comments is not included here; however we have provided the results of running the OCaml code with the traceability comments in "OCaml Results.pdf".

Note: The OCaml code includes many more examples than are in the book. When the OCaml code was run, there were many additional examples that were commented out. We are working toward adding all of the examples to the PDF document. If you look at the OCaml run output for unif.ml you will notice that the traceability comment for unify.p003 is missing. This is because the code for unify.p003 was commented out when the run occurred. Also, there were no tests in the original OCaml code for the lib module. As this is a port, many new tests were created for the lib module in the F# code, these test currently have not been created for the OCaml version for comparison, but we are working toward that also.

Build Status

The build status above is via travis-ci. More info on travis-ci can be fount at the travis-ci Github page.

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