For a project, I require that my users are able to provide a simple function as a string, and I want to be able to evaluate it from my program.
Let's say the function signature is always int f(int a, int b)
I want to make it so that my users can provide implementations of f written in a basic language.
For instance:
return a*b;
if(a ==1) return b else return 3
And in my program, I want to be able to parse the user input function to make it a real callable function from my C# code. I only require basic features like basic math stuff and conditionals (not even loops).
Is there a basic "language" that exists that would allow me to interpret such functions defined as input and transform them in callable code?
For instance I think that using Lua for this overkill?
Ideally I want something open-source, with minimal codebase and that is widely known to be of good quality. Does such a thing exist? Ideally it should also have implementations in several languages, like C# and Java.
I know the temptation to write my own parser is high but I feel like there must be an out-of-the-box solution for my problem
If you only need math functions you can just have take in a string an eval it in a data table then return the results.
using System.Data;
//string mathExp = "3 * (2+4)"
public string ShiestyEval(string mathExp)
{
DataTable dt = new DataTable();
var v = dt.Compute(mathExp,"");
return v;
}
To get it to be callable from your application, you need to provide an interface in which they can eval there methods, or if you wanna do some shady hack and generate an assembly though Il, and load it you can, it's a little vague on how you want to use it
you can also checkout JSharp
I don't think there is a simple way to do it. But there are some solutions that are easy to use. I really like Irony had a lot of fun with it. But it's a bit abandoned. You can use ANTLR but this one is not simple. You might be able to use Roslyn Api to do what you want it all depends. I am not sure how good of a sandbox do you need but you can use C# scripts
This will allow you to do something like this
ScriptState state = CSharpScript.Run("int Times(int x) { return x * x; }");
var fn = state.CreateDelegate>("Times");
var value = fn(5);
Console.Write(value);
Related
I would like to have a C# method which counts the number of cases in a switch statment. (Let's call it CaseCounter). For example, let's say you have this enum and these two methods:
enum Painter
{
Rubens,
Rembrandt,
Vermeer,
Picasso,
Kandinsky
}
int GetYearOfBirth(Painter painter)
{
switch(painter)
{
case Painter.Kandinsky:
return 1866;
case Painter.Picasso:
return 1881;
case Painter.Rembrandt:
return 1606;
case Painter.Rubens:
return 1577;
case Painter.Vermeer:
return 1632;
default:
return 0;
}
}
bool IsModern(Painter painter)
{
switch (painter)
{
case Painter.Kandinsky:
case Painter.Picasso:
return true;
default:
return false;
}
}
Now the following equalities should hold:
CaseCounter("GetYearOfBirth") == 5
CaseCounter("IsModern") == 2
(It is not important whether or not you include the default case in the count. Also, the parameter to CaseCounter doesn't need to be a string; any type which can somehow be used to represent a method will do.)
So, how would you go about implementing CaseCounter? Is it even possible?
--- ADDENDUM (EDIT) ---
Here's a bit of background info in case you're wondering why I asked this question.
I'm maintaining a code base which contains a lot of methods that switch on enums. The methods are spread across various classes; this makes life difficult when an enum is extended (for example, when you add a new Painter). To make maintenance a little bit easier, I have written some unit tests of the following form:
// If this test fails please check that the following methods are still OK:
// MyClass.GetYearOfBirth, MyOtherClass.IsModernAllCases .
// (There is no guarantee that the above list is up-to-date or complete.)
[Test]
public void PainterCountTest()
{
int numberOfMembers = Enum.GetValues(typeof(NotificationID)).Length;
Assert.AreEqual(5, numberOfMembers);
}
(In this example, IsModernAllCases is just a variation of IsModern which refers explicitly to all 5 possible values of the Painter enum.)
This test is better than nothing, but it's clumsy. It would be a little less clumsy if you could write something like this:
[Test]
public void PainterCountTest()
{
int numberOfMembers = Enum.GetValues(typeof(NotificationID)).Length;
int numberOfCases_getYearOfBirth = CaseCounter("MyClass.GetYearOfBirth");
Assert.AreEqual(numberOfCases_getYearOfBirth, numberOfMembers);
int numberOfCases_modern = CaseCounter("MyOtherClass.IsModernAllCases");
Assert.AreEqual(numberOfCases_modern, numberOfMembers);
}
In this scenario, at least you don't have to modify the unit test when you extend the enum.
It should be possible to do with the Roslyn CTP - Microsoft's compiler-as-a-service pre-release product. It has API's to let you inspect C# code and represent it as a tree of instructions. Unfortunately, it is a CTP, which may or may not be a problem for you - just remember it is pre-release software if you try to use it.
If you can't use Roslyn, I think the only way would be to inspect the generated IL. A daunting task, to say the least. I haven't got any sample code for that, but if you want to try it - I would start by looking at the Cecil Mono project, which has some API's for inspecting IL.
You could also MethodInfo.GetMethodBody to get the raw IL bytes and then parse those bytes yourself, but it does require some work to do that.
Also, see this related question.
This can't be done via Reflection, but could be done via the Roslyn CTP. Roslyn provides the tooling required to analyze the source code itself, and determine information about it. You could walk the trees in this code to find the methods containing switch statements, and make a count of individual cases.
Without knowing why you are doing this, it's tough to know if this will be an acceptable approach for you.
Since these C# files are basically just text files on your computer, you could create a separate application that loops through all the files. It would need to recognize methods, count the number of occurrences of case words (that aren't comments), and report the results.
Now, if you need this functionality to run at runtime, within your app, then this approach won't work.
make a find search in visual studio on your case word it is enough to make your refactoring
I've been developing .net console applications using C# and have always just dictated what order parameters must be inserted in so that args[0] is always start date and args[1] is always end date, for example.
however I would like to move over to using named parameters so that any combination of parameters can be sent in any order, such as the typical "-sd" would prefix a start date.
I know I could parse through the args[] looking for "-" and then read the name and look the next position for the accompanying value, but before doing that wanted to see if there was any kind of baked in handling for this rather standard practice.
is there something like this out there already that could do as such:
DateTime startDate = (DateTime)((ConsoleParameters)args[])["sd"]
I'm using C# and .Net 4
There is nothing built into the core framework.
A lot of people think NDesk.Options is useful for this sort of thing. Check out this example (taken directly from the provided link):
string data = null;
bool help = false;
int verbose = 0;
var p = new OptionSet () {
{ "file=", v => data = v },
{ "v|verbose", v => { ++verbose } },
{ "h|?|help", v => help = v != null },
};
List<string> extra = p.Parse (args);
Yes, the "magic" is that this is a common problem and it has been adequately solved. So I recommend using an already written library to handle parsing command line arguments.
CommandLineParser has been great for me. It is reasonably documented and flexible enough for every type of command line argument I've wanted to handle. Plus, it assists with usage documentation.
I will say that I'm not the biggest fan of making a specific class that has to be adorned with attributes to use this library, but it's a minor point considering that it solves my problem. And in reality forcing that attributed class pushes me to keep that class separate from where my app actually retrieves it's settings from and that always seems to be a better design.
You can use NDesk.Options.
There is no such a thing as named parameters. "-sd" is just a choice for a specific application. It can be "/sd" as well. Or "sd=". Or whatever you want.
Since there are no named parameters, there is nothing inside .NET Framework which let you use the "-sd" syntax.
But you can quite easily build your own method to get a set of "named parameters" for your app.
Edit: or, even better, you can use an existing library, like suggested in other answers.
Edit: reading the answer by #Sander Rijken, I see that I was wrong: there were still an implementation of "-sd" syntax in .NET 4.0 before the release. But since it was dropped before the final release, the only ways are still to create your own method or to use an existing library.
I have a simple .NET application which runs as Windows Service.
Say it has one Class
MyClass
{
Run(Destination d)
Walk(Destination d)
Wash(Dishes d)
}
I want to create a console application where I would type simple human readable commands
like
run left
walk right
Pretty much like you do in windows console. I wonder what is the best way to implement this mapping. A straight methods is of course create your own string parser with lots of switch statements but I think if there is better and faster method.
This might be too much for your needs, but I think a robust, flexible way to do it would be creating a couple of meta-data attributes to decorate the classes you might want to call from your shell, something like:
[ScriptableClass]
public class MyClass
{
[ScriptableMethod(typeof (Destination),typeof(int))]
public void Run (Destination f, int distance) {}
}
Then, during your shell's startup, you load your assemblies via reflection and look for types marked with the ScriptableClass attribute. For each of them, you inspect their public methods looking for those marked with the ScriptableMethod attribute, and you can build a dictionary of classes and their scriptable methods (along with info about parameters). For the previous example, you would have a 'MyClass.Run' command you can could from the shell. When received, your scripting engine would create / lookup an instance of the class and execute the method.
The advantage of this method is that you wouldn't have to modify your scripting engine every time you add new classes or modify methods.
You have two options here:
Use reflection to find the class method with the approrpiate name and invoking it.
Use the command pattern, where each command object would have a name and a Run() method. You can easily find the desired command by name, then run it.
UPDATE: Aggreed, there are more than two options (see Guffa's answer). But these two are the ones that make the code clearer IMHO.
Not really. There are other methods, but if they are better is mostly a matter of taste, and for identifying a string you can't do it much faster than a switch.
If all method have the same parameters, you could set up a dictionary of strings and delegates.
Example:
var methods = new Dictionary<string, Action<int>>();
methods.Add("run", n => MyClass.Run(n));
methods.Add("walk", n => MyClass.Walk(n));
methods.Add("wash", n => MyClass.Wash(n));
string cmd = "run";
int param = 42;
if (methods.ContainsKey(cmd)) {
methods[cmd](param);
} else {
Console.WriteLine('Say what, human?');
}
Perhaps a PowerShell CommandLet would be an option, although that would constrain you or your users to PowerShell (which is a superior shell IMO).
I'm looking for the equivalent of C# default keyword, e.g:
public T GetNext()
{
T temp = default(T);
...
Thanks
I found this in a blog: "What does this C# code look like in F#? (part one: expressions and statements)"
C# has an operator called "default"
that returns the zero-initialization
value of a given type:
default(int)
It has limited utility; most commonly you may use
default(T) in a generic. F# has a
similar construct as a library
function:
Unchecked.defaultof<int>
Technically speaking the F# function Unchecked.defaultof<'a> is an equivalent to the default operator in C#. However, I think it is worth noting that defaultof is considered as an unsafe thing in F# and should be used only when it is really necessary (just like using null, which is also discouraged in F#).
In most situations, you can avoid the need for defaultof by using the option<'a> type. It allows you to represent the fact that a value is not available yet.
However, here is a brief example to demonstrate the idea. The following C# code:
T temp = default(T);
// Code that may call: temp = foo()
if (temp == default(T)) temp = bar(arg)
return temp;
Would be probably written like this in F# (using imperative features):
let temp = ref None
// Code that may call: temp := Some(foo())
match !temp with
| None -> bar(arg)
| Some(temp) -> temp
Of course this depends on your specific scenario and in some cases defaultof is the only thing you can do. However, I just wanted to point out that defaultof is used less frequently in F#.
There is a lot of talk about monads these days. I have read a few articles / blog posts, but I can't go far enough with their examples to fully grasp the concept. The reason is that monads are a functional language concept, and thus the examples are in languages I haven't worked with (since I haven't used a functional language in depth). I can't grasp the syntax deeply enough to follow the articles fully ... but I can tell there's something worth understanding there.
However, I know C# pretty well, including lambda expressions and other functional features. I know C# only has a subset of functional features, and so maybe monads can't be expressed in C#.
However, surely it is possible to convey the concept? At least I hope so. Maybe you can present a C# example as a foundation, and then describe what a C# developer would wish he could do from there but can't because the language lacks functional programming features. This would be fantastic, because it would convey the intent and benefits of monads. So here's my question: What is the best explanation you can give of monads to a C# 3 developer?
Thanks!
(EDIT: By the way, I know there are at least 3 "what is a monad" questions already on SO. However, I face the same problem with them ... so this question is needed imo, because of the C#-developer focus. Thanks.)
Most of what you do in programming all day is combining some functions together to build bigger functions from them. Usually you have not only functions in your toolbox but also other things like operators, variable assignments and the like, but generally your program combines together lots of "computations" to bigger computations that will be combined together further.
A monad is some way to do this "combining of computations".
Usually your most basic "operator" to combine two computations together is ;:
a; b
When you say this you mean "first do a, then do b". The result a; b is basically again a computation that can be combined together with more stuff.
This is a simple monad, it is a way of combing small computations to bigger ones. The ; says "do the thing on the left, then do the thing on the right".
Another thing that can be seen as a monad in object oriented languages is the .. Often you find things like this:
a.b().c().d()
The . basically means "evaluate the computation on the left, and then call the method on the right on the result of that". It is another way to combine functions/computations together, a little more complicated than ;. And the concept of chaining things together with . is a monad, since it's a way of combining two computations together to a new computation.
Another fairly common monad, that has no special syntax, is this pattern:
rv = socket.bind(address, port);
if (rv == -1)
return -1;
rv = socket.connect(...);
if (rv == -1)
return -1;
rv = socket.send(...);
if (rv == -1)
return -1;
A return value of -1 indicates failure, but there is no real way to abstract out this error checking, even if you have lots of API-calls that you need to combine in this fashion. This is basically just another monad that combines the function calls by the rule "if the function on the left returned -1, do return -1 ourselves, otherwise call the function on the right". If we had an operator >>= that did this thing we could simply write:
socket.bind(...) >>= socket.connect(...) >>= socket.send(...)
It would make things more readable and help to abstract out our special way of combining functions, so that we don't need to repeat ourselves over and over again.
And there are many more ways to combine functions/computations that are useful as a general pattern and can be abstracted in a monad, enabling the user of the monad to write much more concise and clear code, since all the book-keeping and management of the used functions is done in the monad.
For example the above >>= could be extended to "do the error checking and then call the right side on the socket that we got as input", so that we don't need to explicitly specify socket lots of times:
new socket() >>= bind(...) >>= connect(...) >>= send(...);
The formal definition is a bit more complicated since you have to worry about how to get the result of one function as an input to the next one, if that function needs that input and since you want to make sure that the functions you combine fit into the way you try to combine them in your monad. But the basic concept is just that you formalize different ways to combine functions together.
It has been a year since I posted this question. After posting it, I delved into Haskell for a couple of months. I enjoyed it tremendously, but I placed it aside just as I was ready to delve into Monads. I went back to work and focused on the technologies my project required.
And last night, I came and re-read these responses. Most importantly, I re-read the specific C# example in the text comments of the Brian Beckman video someone mentions above. It was so completely clear and illuminating that I’ve decided to post it directly here.
Because of this comment, not only do I feel like I understand exactly what Monads are … I realize I’ve actually written some things in C# that are Monads … or at least very close, and striving to solve the same problems.
So, here’s the comment – this is all a direct quote from the comment here by sylvan:
This is pretty cool. It's a bit abstract though. I can imagine people
who don't know what monads are already get confused due to the lack of
real examples.
So let me try to comply, and just to be really clear I'll do an
example in C#, even though it will look ugly. I'll add the equivalent
Haskell at the end and show you the cool Haskell syntactic sugar which
is where, IMO, monads really start getting useful.
Okay, so one of the easiest Monads is called the "Maybe monad" in
Haskell. In C# the Maybe type is called Nullable<T>. It's basically
a tiny class that just encapsulates the concept of a value that is
either valid and has a value, or is "null" and has no value.
A useful thing to stick inside a monad for combining values of this
type is the notion of failure. I.e. we want to be able to look at
multiple nullable values and return null as soon as any one of them
is null. This could be useful if you, for example, look up lots of
keys in a dictionary or something, and at the end you want to process
all of the results and combine them somehow, but if any of the keys
are not in the dictionary, you want to return null for the whole
thing. It would be tedious to manually have to check each lookup for
null and return, so we can hide this checking inside the bind
operator (which is sort of the point of monads, we hide book-keeping
in the bind operator which makes the code easier to use since we can
forget about the details).
Here's the program that motivates the whole thing (I'll define the
Bind later, this is just to show you why it's nice).
class Program
{
static Nullable<int> f(){ return 4; }
static Nullable<int> g(){ return 7; }
static Nullable<int> h(){ return 9; }
static void Main(string[] args)
{
Nullable<int> z =
f().Bind( fval =>
g().Bind( gval =>
h().Bind( hval =>
new Nullable<int>( fval + gval + hval ))));
Console.WriteLine(
"z = {0}", z.HasValue ? z.Value.ToString() : "null" );
Console.WriteLine("Press any key to continue...");
Console.ReadKey();
}
}
Now, ignore for a moment that there already is support for doing this
for Nullable in C# (you can add nullable ints together and you get
null if either is null). Let's pretend that there is no such feature,
and it's just a user-defined class with no special magic. The point is
that we can use the Bind function to bind a variable to the contents
of our Nullable value and then pretend that there's nothing strange
going on, and use them like normal ints and just add them together. We
wrap the result in a nullable at the end, and that nullable will
either be null (if any of f, g or h returns null) or it will be
the result of summing f, g, and h together. (this is analogous
of how we can bind a row in a database to a variable in LINQ, and do
stuff with it, safe in the knowledge that the Bind operator will
make sure that the variable will only ever be passed valid row
values).
You can play with this and change any of f, g, and h to return
null and you will see that the whole thing will return null.
So clearly the bind operator has to do this checking for us, and bail
out returning null if it encounters a null value, and otherwise pass
along the value inside the Nullable structure into the lambda.
Here's the Bind operator:
public static Nullable<B> Bind<A,B>( this Nullable<A> a, Func<A,Nullable<B>> f )
where B : struct
where A : struct
{
return a.HasValue ? f(a.Value) : null;
}
The types here are just like in the video. It takes an M a
(Nullable<A> in C# syntax for this case), and a function from a to
M b (Func<A, Nullable<B>> in C# syntax), and it returns an M b
(Nullable<B>).
The code simply checks if the nullable contains a value and if so
extracts it and passes it onto the function, else it just returns
null. This means that the Bind operator will handle all the
null-checking logic for us. If and only if the value that we call
Bind on is non-null then that value will be "passed along" to the
lambda function, else we bail out early and the whole expression is
null. This allows the code that we write using the monad to be
entirely free of this null-checking behaviour, we just use Bind and
get a variable bound to the value inside the monadic value (fval,
gval and hval in the example code) and we can use them safe in the
knowledge that Bind will take care of checking them for null before
passing them along.
There are other examples of things you can do with a monad. For
example you can make the Bind operator take care of an input stream
of characters, and use it to write parser combinators. Each parser
combinator can then be completely oblivious to things like
back-tracking, parser failures etc., and just combine smaller parsers
together as if things would never go wrong, safe in the knowledge that
a clever implementation of Bind sorts out all the logic behind the
difficult bits. Then later on maybe someone adds logging to the monad,
but the code using the monad doesn't change, because all the magic
happens in the definition of the Bind operator, the rest of the code
is unchanged.
Finally, here's the implementation of the same code in Haskell (--
begins a comment line).
-- Here's the data type, it's either nothing, or "Just" a value
-- this is in the standard library
data Maybe a = Nothing | Just a
-- The bind operator for Nothing
Nothing >>= f = Nothing
-- The bind operator for Just x
Just x >>= f = f x
-- the "unit", called "return"
return = Just
-- The sample code using the lambda syntax
-- that Brian showed
z = f >>= ( \fval ->
g >>= ( \gval ->
h >>= ( \hval -> return (fval+gval+hval ) ) ) )
-- The following is exactly the same as the three lines above
z2 = do
fval <- f
gval <- g
hval <- h
return (fval+gval+hval)
As you can see the nice do notation at the end makes it look like
straight imperative code. And indeed this is by design. Monads can be
used to encapsulate all the useful stuff in imperative programming
(mutable state, IO etc.) and used using this nice imperative-like
syntax, but behind the curtains, it's all just monads and a clever
implementation of the bind operator! The cool thing is that you can
implement your own monads by implementing >>= and return. And if
you do so those monads will also be able to use the do notation,
which means you can basically write your own little languages by just
defining two functions!
A monad is essentially deferred processing. If you are trying to write code that has side effects (e.g. I/O) in a language that does not permit them, and only allows pure computation, one dodge is to say, "Ok, I know you won't do side effects for me, but can you please compute what would happen if you did?"
It's sort of cheating.
Now, that explanation will help you understand the big picture intent of monads, but the devil is in the details. How exactly do you compute the consequences? Sometimes, it isn't pretty.
The best way to give an overview of the how for someone used to imperative programming is to say that it puts you in a DSL wherein operations that look syntactically like what you are used to outside the monad are used instead to build a function that would do what you want if you could (for example) write to an output file. Almost (but not really) as if you were building code in a string to later be eval'd.
You can think of a monad as a C# interface that classes have to implement. This is a pragmatic answer that ignores all the category theoretical math behind why you'd want to choose to have these declarations in your interface and ignores all the reasons why you'd want to have monads in a language that tries to avoid side effects, but I found it to be a good start as someone who understands (C#) interfaces.
See my answer to "What is a monad?"
It begins with a motivating example, works through the example, derives an example of a monad, and formally defines "monad".
It assumes no knowledge of functional programming and it uses pseudocode with function(argument) := expression syntax with the simplest possible expressions.
This C# program is an implementation of the pseudocode monad. (For reference: M is the type constructor, feed is the "bind" operation, and wrap is the "return" operation.)
using System.IO;
using System;
class Program
{
public class M<A>
{
public A val;
public string messages;
}
public static M<B> feed<A, B>(Func<A, M<B>> f, M<A> x)
{
M<B> m = f(x.val);
m.messages = x.messages + m.messages;
return m;
}
public static M<A> wrap<A>(A x)
{
M<A> m = new M<A>();
m.val = x;
m.messages = "";
return m;
}
public class T {};
public class U {};
public class V {};
public static M<U> g(V x)
{
M<U> m = new M<U>();
m.messages = "called g.\n";
return m;
}
public static M<T> f(U x)
{
M<T> m = new M<T>();
m.messages = "called f.\n";
return m;
}
static void Main()
{
V x = new V();
M<T> m = feed<U, T>(f, feed(g, wrap<V>(x)));
Console.Write(m.messages);
}
}