Just as my title . I want my VS to auto indent for me like in VBNET . Please help.
As far as I know, the F# language integration doesn't support this feature.
Also, automatic formatting is not as useful in F# as it is in Visual Basic. In VB, the formatting is not really important (so you can write code with crazy indentation and the formatter can fix it). In F#, the indenation (partly) determines what code means, so you need to write correctly indented code (although I agree that the automatic formatting could make the code more consistent).
In principle it should be possible to implement this feature as a Visual Studio plugin using the open source release of F#. There is a similar plugin that adds colors for nested expressions by Brian, so that could be used as an inspiration, but it's definitely not something I could write in the answer box :-).
Sadly, the indentation-sensitive syntax that F# inherited from languages like Haskell makes it impossible to auto-indent. This is actually my only major gripe with the F# language because, in addition to making it impossible to implement professional tools like auto-indenters, it renders programs fragile in the absence of correct indentation which means an accidental change in whitespace can silently corrupt a program and cut-and-paste (e.g. from blogs) is prone to breaking or corrupting programs. F# almost always screws up if you feed it OCaml code, partly because it cannot handle tabs.
The damn crying shame is that OCaml already got this right by providing a concise unambiguous syntax and powerful tools. For example, you can autoindent any definition by pressing ALT+Q in Emacs. This makes it much easier to manipulate OCaml code and can be an enormous time saver. I often find myself trawling through F# code trying to reindent it by hand, having to read the code in detail and understand the algorithm just to indent it is seriously frustrating. Having done this many times, I can also state quite confidently that the verbosity savings of the #light syntax are insignificant. In fact, F# is almost always more verbose than OCaml in practice.
I prefer to pour cold water all over this question. In principle, it's impossible to provide an auto-formatter for a whitespace-significant language.
(Pragmatically, you could add a few small niceties to the editor, e.g. if you type a line of code that starts with if and ends with the matching then and press enter, the editor could get smart and also insert the next indent so you don't also have to press tab. But this is a far cry from auto-formatting, which I think would be wrong-headed to even attempt.)
Related
In researching a topic related to programming I came across a pointfree refactoring tool for Haskell in the lambdabot and was wondering if F# can be refactored into a pointfree style?
I am not advocating the use of pointfree style, but see it as a means to better comprehend a function.
Note: pad answered an earlier version of this question, but I reworded this question as the answer is of value to others learning and using F# and I did not want this to be deleted because of some close votes.
Note: Just because I changed the question, don't take the answer to mean that one can not code in a point free style using F#. It can be done in many cases but there are restrictions you have to follow.
Short answer
No.
Long answer
There are a few things in F# that make such a tool impractical. (1) Due to .NET interop, F# code often has side effects and automatic code transformation becomes really difficult when side effects come in play. It's not the case with Haskell; equational reasoning is much easier in Haskell and you can rewrite left-hand sides by right-hand sides without altering their evaluations. (2) Point-free programming in F# is limited by value restriction. I'm not sure you can do code transformation aggressively without hitting this issue.
I think it's more practical to assume that F# code is pure and value restriction doesn't occur in particular cases in order that we can give users some hints. Users can apply suggestions discretely after evaluating that the suggestions are actually correct. It's closer to HLint approach than the one you referred to. FSharpLint has added some linting rules in this direction.
HLint is a command line static analysis tool for Haskell code, that even suggests the appropriate refactored version of the code. Anyone know of similar command line tools for linting F# code?
Short answer:
No, there is no such tool yet.
Long answer:
Let's discuss how to build it then.
I did some background research which might be useful.
References
There are a few lint tools in functional languages, which can be used as sources of inspiration. However, they tend to go to different directions.
HLint is an advanced tool and its refactoring capability is amazing. Refactoring suggestion is more tricky in F# due to (1) F# code might have side effects so equational reasoning is unsound (2) When doing point-free transformation, value restriction could eliminate some good suggestions. If we accept false positives, it might become a bit easier.
In Scala's world, you have Wart Remover and Scala Style. The former focuses on common functional programming mistakes in Scala. The latter has its focus on human errors and inconsistencies (e.g. naming, convention, etc.). I guess Wart Remover is more relevant to F# as it is a functional-first programming language. However, a style checker tool is useful on a big code base with multiple developers.
The most relevant lint tool for F# is probably OCaml's style checker, Mascot. It has a big and extensible rule set. Many of these rules are applicable to F# with minor adaptation.
Resources (and the lack thereof)
What we have:
F# compiler is on GitHub. The relevant component - F# compiler service has a NuGet package so bootstrapping is easy. F# compiler source code is a good resource since F# compiler's warnings are very good and informative.
There are recent works using F# compiler e.g. language binding, refactoring, code formatting, etc. so we have experience to build up on.
We have other lint tools to learn from.
What we don't have:
Good documentation on recommended styles and practices in F# is missing. The design guideline has been useful, but it isn't complete enough.
Building lint tool is time-consuming and difficult. Even with simple and single-purpose tool like Fantomas; it takes a lot of time to process F#'s ASTs in a correct way.
To sum up, if we define a right scope, creating a simple yet useful tool for F# is within reach.
Updates
There is an actively-developing linter for F# available at https://github.com/duckmatt/FSharpLint. It seems that my analysis is not too far off :).
What is the best way to use F# to parse an AST to build an interpreter? There are plenty of F# examples for trivial syntax (basic arithmatical operations) but I can't seem to find anything for languages with much larger ranges of features.
Discriminated unions look to be incrediably useful but how would you go about constructing one with large number of options? Is it better to define the types (say addition, subtraction, conditionals, control flow) elsewhere and just bring them together as predefined types in the union?
Or have I missed some far more effective means of writing interpreters? Is having an eval function for each type more effective, or perhaps using monads?
Thanks in advance
Discriminated unions look to be
incrediably useful but how would you
go about constructing one with large
number of options? Is it better to
define the types (say addition,
subtraction, conditionals, control
flow) elsewhere and just bring them
together as predefined types in the
union?
I am not sure what you are asking here; even with a large number of options, DUs still are simple to define. See e.g. this blog entry for a tiny language's DU structure (as well as a more general discussion about writing tree transforms). It's fine to have a DU with many more cases, and common in compilers/interpreters to use such a representation.
As for parsing, I prefer monadic parser combinators; check out FParsec or see this old blog entry. After using such parser combinators, I can never go back to anything like lex/yacc/ANTLR - external DSLs seem so primitive in comparison.
(EDIT: The 'tiny arithmetic examples' you have found are probably pretty much representative of what larger solutions looks like as well. The 'toy' examples usually show off the right architecture.)
You should take a copy of Robert Pickering's "Beginning F#".
The chapter 13, "Parsing Text", contains an example with FsLex and FsYacc, as suggested by Noldorin.
Other than that, in the same book, chapter 12, the author explains how to build an actual simple compiler for an arithmetic language he proposes. Very enlightening. The most important part is what you are looking for: the AST parser.
Good luck.
I second Brian's suggestion to take a look at FParsec. If you're interested in doing things the old-school way with FsLex and FsYacc, one place to look for how to parse a non-trivial language is the F# source itself. See the source\fsharp\FSharp.Compiler directory in the distribution.
You may be interesting in checking out the Lexing and Parsing section of the F# WikiBook. The F# PowerPack library contains the FsLex and FsYacc tools, which asssist greatly with this. The WikiBook guide is a good way to get started with this.
Beyond that, you will need to think about how you actually want to execute the code from the AST form, which is common the design of both compilers and interpreters. This is generally considered the easier part however, and there are lots of general resources on compilers/interpreter out there that should provide information on this.
I haven't done an interpreter myself. Hope the following helps:)
Here's a compiler course taught at Yale using ML, which you might find useful. The lecture notes are very concise (short) and informative. You can follow the first few lecture notes and the assignments. As you know F#, you won't have problem reading ML programs.
Btw, the professor was a student of A. Appel, who is the creator of SML implementation. So from these notes, you also get the most natural way to write a compiler/interpreter in ML family language.
This is an excellent example of a complete Small Basic implementation with F# and FParsec. It includes even IL compiler. The whole code is very accessible and is accompanied by a series of blog post from the author at http://trelford.com/blog/
I have been programming since 1999 for work and fun. I want to learn new things, and lately I've been focused on parsing, as a large part of my job is reading, integrating and analyzing data. I also have a large number of repetitive tasks that I think I could express in very simple domain-specific languages if the overhead was low enough. I have a few questions about the subject.
Most of my current parsing code don't define a formal grammar. I usually hack something together in my language of choice because that's easy, I know how to do it and I can write that code very fast. It's also easy for other people I work with to maintain. What are the advantages and disadvantages of defining a grammar and generating a real parser (as one would do with ANTLR or YACC) to parse things compared with the hacks that most programmers used to write parsers?
What are the best parser generation tools for writing grammar-based parsers in C++, Perl and Ruby? I've looked at ANTLR and haven't found much about using ANTLRv3 with a C++ target, but otherwise that looks interesting. What are the other tools that are similar to ANTLR that I should be reading about?
What are the canonical books and articles that someone interested in learning more about parsing? A course in compilers unfortunately wasn't part of my education, so basic material is very welcome. I've heard great things about the Dragon Book, but what else is out there?
On 1., I would say the main advantage is maintainability -- making a little change to the language just means making a correspondingly-small change to the grammar, rather than minutely hacking through the various spots in the code that may have something to do with what you want changed... orders of magnitude better productivity and smaller risk of bugs.
On 2. and 3., I can't suggest much beyond what you already found (I mostly use Python and pyparsing, and could comment from experience on many Python-centered parse frameworks, but for C++ I mostly use good old yacc or bison anyway, and my old gnarled copy of the Dragon Book -- not the latest edition, actually -- is all I keep at my side for the purpose...).
Here's my take on your (very good) questions:
I think a parser benefits most from non-trivial situations where a grammar actually exists. You have to know about how parsers and grammars work to think of that technique, and not every developer does.
lex/yacc are older Unix tools that might be usable for you as a C++ developer. Maybe Bison as well.
ANTRL and its attendant book are very good. "Writing Compilers and Interpreters" has C++ examples which you might like.
The GoF Interpreter pattern is another technique for writing "little languages". Take a look at that.
Let's Build A Compiler is a step-by-step tutorial on how to write a simple compiler. The code is written in Delphi (Pascal), but it's basic enough to easily translate into most other languages.
I would have a serious look at monadic combinator-based parsing (which often also deals with lexical analysis) in Haskell. I found it quite an eye opener; it's amazing how easily you can build a parser from scratch using this method. It's so easy, in fact, that it's often faster to write your own parser than it is to try to use existing libraries.
The most famous example is probably Parsec which has a good user guide that explains how to use it. There is a list of ports of this library to other languages (including C++ and Ruby) listed on the Parsec page of the Haskell wiki, though I'm not familiar with them and so I can't say how close they are to using Parsec in Haskell.
If you want to learn how these work internally and how to write your own, I recommend starting with Chapter 8 ("Functional Parsers") from Graham Hutton's Programming in Haskell. Once you understand that chapter well (which will probably take several readings), you'll be set.
In perl, the Parse::RecDescent modules is the first place to start. Add tutorial to the module name and Google should be able to find plenty of tutorials to get you started.
Defining a grammar using BNF, EBNF or something similar, is easier and later on you will have a better time maintaining it. Also, you can find a lot of examples of grammar definitions. Last but not least, if you are going to talk about your grammar to someone else on the field, it is better if you are both speaking the same language (BNF, EBNF etc.).
Writing your own parsing code is like reinventing the wheel and is prone to errors. It is also less maintainable. Of course, it can be more flexible, and for small projects it might also be a good choice, but using an existing parser generator that takes a grammar and spits out the code should cover most of our needs.
For C++ I would also suggest lex/yacc. For Ruby this looks like a decent choice: Coco/R(uby)
Funny timing: I spent lots of this morning wondering about state machines and parsers, and trying to figure out how I could learn more about them.
For 2, you might take a look at Ragel (it's good for C++ and Ruby).
Here's a tutorial on a self-contained (10 pages!), completely portable compiler-compiler
which can be used to design and implement "low overhead" DSLs very quickly:
http://www.bayfronttechnologies.com/mc_tutorial.html
This site walks you through Val Schorre's 1964 paper on MetaII.
Yes, 1964. And it is amazing. This is how I learned about compilers
back in 1970.
I have recently spent several years translating legacy FORTRAN into Java. Prior to that, I found myself translating FORTRAN into C (for which I wrote a simple translation tool). After all this work, I find myself wondering how many others are doing similar language-to-language translations and whether an automated way of doing so would be beneficial.
I know about F2C, For_C, F2J and others, as well as some of the translation sites, but none seem to be all that successful. Having seen output from For_C, I can see why it just hasn't taken off. While it is technically correct, it is very difficult to maintain.
So, I guess what I am wondering is if there were are tool that produced more maintainable, more grok-able code than the code I have seen, would developers use it? Or are developers as jaded as many posts seem to indicate and unwilling to use generated code as it could never be as good as their manually translated code?
In short, no. Obviously time restraints necessitate it sometimes, but...
Rarely is code written in one language going to translate well to another - every language has certain ways of doing things that are more suited to the constructs available / common libraries / etc.
Consider for example a program written in C as compared to something written in Python - certainly you can write for loops and iterate through things in Python just as easily as you can in C, but it is much simpler to use list comprehensions and take advantage of the features the language provides.
I'd be surprised to see an example of a reasonably sized program written in any language that could be translated into 'correct', well-maintainable code in any other.
This was already covered to some extent in Conversion of Fortran 77 code to C++, but I'll take a stab at it here.
I think there's a lot of time wasted translating legacy code to new languages. It takes a phenomenal amount of time and energy to do, and you introduce new bugs when you do it.
Joel mentioned why rewriting from scratch is a horrible idea in Things you Should Never do Part I, and though I realize that translating something to a new language isn't quite the same as rewriting from scratch, I claim it's close enough:
Automated translation tools aren't wonderful because you don't get anything maintainable out of them. You pretty much have to know the old code to understand the new code, and then what have you gained?
To port something manually, you have to know how the code works to do it well. Rewriting code is seldom done by the original developers, so you seldom get people who understand everything that's going on to do the rewrite. I worked at a company where an outsource team was hired to translate an entire website backend from ColdFusion to JSP. That project kept getting delayed and delayed because the port team didn't know the code at all. Our guys never quite liked their design, and they never quite got it right, so there was constant iteration as everyone worked out all the issues that were solved in the original code. Then, the porting itself took forever.
You also need to be familiar with really technical inconsistencies between languages. People who are very familiar with two languages are rare.
For Fortran specifically, I now work at a place where there are millions of lines of legacy Fortran code, and no one here is about to rewrite it. There's just too much risk. Old bugs would have to be re-fixed, and there are hundreds of man-years that went into working out the math. Nobody wants to introduce those kinds of bugs, and it's probably downright unsafe to do it.
Instead of porting, we have hybrid codes. After all, you can link Fortran and C/C++, and if you make a C interface around your Fortran code, you can call it from Java. Modern codes here have C/C++ components that make calls into old Fortran routines, and if you do it this way you get the added benefit that Fortran compilers are screaming fast, so the old code continues to run as fast as it ever did.
I think the best way to handle this is to do any porting you need to do incrementally. Make a lightweight interface around your old fortran code and call the pieces you need, but only port things as you need them in the new part. There are also component frameworks for integrating multi-language applications that can make this easier, but you can check out Conversion of Fortran 77 code to C++ for more on that.
Since programming is hard, no such tool can really exist.
If it was trivial to change one language into another, the idea of "compiler" would be moot. You'd just map the language you liked into the language of the hardware, press the button and be done.
However, it's never that simple. Each VM, each language, each API library adds nuances that are just impossible to automate.
" I can see why it just hasn't taken off. While it is technically correct, it is very difficult to maintain."
Correct for F2C as well as Fortran to machine language. The object code generated from most compilers can't easily be read by people. Either it's cruddy or it's highly optimized. Either way, it doesn't look a thing like an expert human would write in the assembler language for that hardware.
If only compiling could be reduced to some XSLT-like transformations that preserved the clarity of the old language in the new language. If there was only some universal Lingua Franca of computing that would be the Rosetta Stone of programming.
Until someone invents that Lingua Franca of computing, every language translation job will be hard and will lead to code that's "difficult to maintain" in the new language.
I've used f2c, and I agree with whoever wanted to name it cc2fc instead. It isn't a way of transforming Fortran into anything vaguely usable as C. It's a way of taking a C compiler and making a Fortran compiler out of it.
It did work just fine at taking that Fortran code and turning it (through C) to a Macintosh library I could call from Macintosh Common Lisp. Those were the days.