In a previous post I described the basic principles of PetitParser and gave some introductory examples. In this blog post I am going to present a way to define more complicated grammars. We continue where we left off the last time, with the expression grammar.
Writing parsers as a script as we did in the previous post can be cumbersome, especially if grammar productions that are mutually recursive and refer to each other in complicated ways. Furthermore a grammar specified in a single script makes it unnecessary hard to reuse specific parts of that grammar. Luckily there is PPCompositeParser to the rescue.
Defining the Grammar
As an example let’s create a composite parser using the same expression grammar we built in the last blog post:
Every production in ExpressionGrammar is specified as a method that returns its parser. Productions refer to each other by reading the respective instance variable of the same name. This is important to be able to create recursive grammars. The instance variables themselves are typically not written to as PetitParser takes care to initialize them for you automatically.
Next we define the productions term, prod, and prim. Contrary to our previous implementation we do not define the production actions yet; and we factor out the parts for addition (add), multiplication (mul), and parenthesis (parens) into separate productions. This will give us better reusability later on. We let Pharo automatically add the necessary instance variables as we refer to them for the first time.
Now that we have defined a grammar we can reuse this definition to implement an evaluator. To do this we create a subclass of ExpressionGrammar called ExpressionEvaluator
For those that missed my PhD defense, below the video recording of the event. Thanks go to Tudor Gîrba and Jorge Ressia for recording it. The slides can be found on SlideShare.
Dynamic Language Embedding With Homogeneous Tool Support
Domain-specific languages (DSLs) are increasingly used as embedded languages within general-purpose host languages. DSLs provide a compact, dedicated syntax for specifying parts of an application related to specialized domains. Unfortunately, such language extensions typically do not integrate well with existing development tools. Editors, compilers and debuggers are either unaware of the extensions, or must be adapted at a non-trivial cost. Furthermore, these embedded languages typically conflict with the grammar of the host language and make it difficult to write hybrid code; few mechanisms exist to control the scope and usage of multiple tightly interconnected embedded languages.
In this dissertation we present Helvetia, a novel approach to embed languages into an existing host language by leveraging the underlying representation of the host language used by these tools. We introduce Language Boxes, an approach that offers a simple, modular mechanism to encapsulate (i) compositional changes to the host language, (ii) transformations to address various concerns such as compilation and syntax highlighting, and (iii) scoping rules to control visibility of fine-grained language changes. We describe the design and implementation of Helvetia and Language Boxes, discuss the required infrastructure of a host language enabling language embedding, and validate our approach by case studies that demonstrate different ways to extend or adapt the host language syntax and semantics.
The defense will take place on Wednesday, October 20, 2010 at 16h15 in Room 001, Engehaldenstrasse 8, Bern. For directions please see http://www.iam.unibe.ch/en/contact.
There is a repeated confusion on how merging in Monticello works. This post tries to clear up the situation. In the following examples we always start from the same situation displayed below. P is the name of the package we are working with and the number behind P.1 signifies a specific version of that package. The Working Copy is the code we currently have in our Smalltalk image.
This shows us that we currently have all the changes from P.0, P.1, P.2 and P.3 in our image. P.6 and P.7 is a separate branch that we do not have in our image.
Load
What happens if we load P.7? If there are any changes in our Working Copy we lose those changes after a warning. Afterwards P.7 is loaded and a new Working Copy is ready to be changed. After this load operation we end up with the following situation:
What happens if we merge P.7? First of all Monticello tries to figure out the closest common ancestor of the Working Copy and P.7. In our examples this is obviously P.1.
Before performing the actual merge Monticello needs to load the code in P.1 into memory. This can cause an error if P.1 has been moved or deleted from its original repository. If you run into troubles, you need to make sure that P.1 is in a repository known to Monticello, otherwise an automatic merge cannot be performed.
To perform the actual merge Monticello calculates the delta between the common ancestor and the two versions to be merged:
D.1 is the delta from P.1 to Working Copy. D.1 is also called the local change, because it is the delta from the common ancestor to the working copy.
D.2 is the delta from P.1 to P.7. D.2 is also called the remote change, because it is the delta from the common ancestor to the version you merge.
Monticello then inspects the two deltas D.1 and D.2. Source elements (class definitions, method definitions, variable definitions, etc.) that are added, removed or changed in both deltas are a conflict. In the merge browser the user needs to resolve these conflicts before proceeding:
The Keep button chooses the remote change from D.2 and ignores the change in D.1.
The Reject button chooses the local change from D.1 and ignores the change in D.2.
Next Monticello creates a patch-set of D.2 with the conflicting elements resolved as specified by the user. It then applies this patch-set onto the working copy and adopts the ancestry as follows:
The Working Copy has now two ancestors P.3 and P.7. In most cases the user will then commit the merged version to the repository. Afterwards the ancestry looks like this:
The merge operation performed by Monticello is called a three-way-merge, because it takes three versions into account: the working copy, the version to merge, and the common ancestor of the two versions.
Manual Merge
What happens if we start from the same situation, but the version of the closest common ancestor P.1 cannot be found. Being able to load P.0 doesn’t help here, because that would cause all the changes from P.0 to P.1 to conflict. Furthermore, if any of the changes between P.0 to P.1 were undone in either of the branches these changes would be lost.
What can we do if the common ancestor is missing? The simples thing is to do a manual merge. This means we calculate the changes from the Working Copy to P.7. This gives us a combined delta D.3 that undoes the changes from P.1 to Working Copy and that adds the changes from P.1 to P.7. Unfortunately Monticello cannot tell us from which branch these changes are coming from, so we have to go through D.3 one-by-one and apply the changes that we think belong to the delta of P.1 to P.7. At the end of the manual merge we can tell Monticello to adopt the version P.7, so that the ancestry looks again like this:
In my recent post I’ve mentioned that the Filesystem library can work on different kinds of filesystems. In this post I am going to walk you through the supported filesystems one by one.
Before you proceed with this hands-on blog post, please update to a patched and unofficial version of the Filesystem package. In the meantime some bugs and minor issues got fixed that are not integrated into the official release yet.
The Disk Filesystem is implemented in FSDiskFilesystem and its platform specific subclasses. As we have seen in the last post the singleton filesystem instance for the current platform can be retrieved using:
disk:=FSDiskFilesystemcurrent.
Subclasses of FSFilesystem implement many methods, but we should resist from calling most of them directly. These methods implement the low-level behavior of the respective file-systems and are private to the framework.
As we have learned in the previous post we should only work with references. Filesystem instances know two methods that return an FSReference object. This is true not only for the disk filesystem, but also for all other filesystem types presented later on:
diskroot." a reference to the root directory " diskworking." a reference to the working directory "
Given a reference we can navigate to another reference in the filesystem with the method #resolve:. Resolving works similar the command cd (change directory) on Unix and Windows and returns a new reference to a file or directory. Print the result of evaluating the following expressions:
diskworkingresolve:'/'." the same as 'disk root' " diskworkingresolve:'.'." the same as 'disk working' " diskworkingresolve:'/home/renggli'." an absolute path to a directory or file " diskworkingresolve:'../bar'." a relative path from the working directory "
Note that the message #resolve: is also understood by FSFilesystem itself. Do not call this method though, it is private and does not return an FSReference as you might expect.
Memory Filesystem
The memory filesystem is another simple filesystem type. Think of it as a virtual in-memory filesystem, very much like a RAM disk that lives in your Smalltalk image. The use of a memory filesystem can be very convenient for testing your filesystem code, because it does not pollute your hard disk and is garbage collected as soon as you do not reference it any longer. To instantiate a memory filesystem evaluate:
memory:=FSMemoryFilesystemnew.
On this filesystem you can do everything you learned before. A memory filesystem is initially empty:
memoryrootchildrensize." --> 0 "
To create a file we can use the same techniques we learned previously:
The above code copies all the files in the package cache of your Pharo installation to the memory filesystem. Before you try it out make sure that you don’t have your MP3 collection in that directory, otherwise your image might blow up.
In my case I have now 64 files in the memory filesystem:
memoryrootchildrensize." --> 64 "
As you would expect we can perform other operations on our virtual filesystem, for example delete all the files that start with the letter F:
As you see, there is nothing special about a memory filesystem. It behaves and understands exactly the same messages as the disk filesystem does.
ZIP Filesystem
The ZIP filesystem represents a ZIP Archive that resides on another filesystem. To create a new archive instantiate the ZIP filesystem with a reference of a ZIP archive:
Contrary to other filesystems a ZIP filesystem needs to be opened (and closed) explicitly:
zipopen.
Apart from that, the ZIP filesystem behaves exactly the same way as the other filesystems we learned up to now. To copy the contents of the memory filesystem to the ZIP archive we can evaluate the following code:
To flush the ZIP archive to the underlying filesystem we simply close it:
zipclose.
This is a convenient way to access archives. Again your code does not have to worry about the details of this particular filesystem, but transparently accesses and modifies it using references.
cURL Filesystem
The cURL filesystem is an experimental extension to the Fileystem framework. It uses the cURL plugin written by Danil Osipchuk to work with filesystems that can be accessed through FTP, FTPS, HTTP, HTTPS, SCP, SFTP, and TFTP.
What about downloading the latest cURL plugin for the Mac VM from within Pharo? To do this we can connect to the directory with the latest experimental code of John McIntosh:
With the resulting filesystem you can do all things you already know. If you are not authenticated however, it is unlikely that you are allowed to write (or upload) to the server. Note that currently enumerating the contents of a directory only works for FTP and SFTP servers. Due to limitations of the CurlPlugin it is furthermore not possible to create directories, delete or rename files. Hopefully that will be fixed sometime soon in the plugin code.
ftpworkingchildren.
To download the curl plugin and save it to your hard disk you can use: