Related
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);
I've recently been coding a lot in both Objective C while also working on several C# projects. In this process, I've found that I miss things in both directions.
In particular, when I code in C# I find I miss the short null check syntax of Objective C.
Why do you suppose in C# you can't check an object for null with a syntax like:
if (maybeNullObject) // works in Objective C, but not C# :(
{
...
}
I agree that if (maybeNullObject != null) is a more verbose / clear syntax, but it feels not only tedious to write it out in code all the time but overly verbose. In addition, I believe the if (maybeNullObject) syntax is generally understood (Javascript, Obj C, and I assume others) by most developers.
I throw this out as a question assuming that perhaps there is a specific reason C# disallows the if (maybeNullObject) syntax. I would think however that the compiler could easily convert an object expression such as if (maybeNullObject) automatically (or automagically) to if (maybeNullObject != null).
Great reference to this question is How an idea becomes a C# language feature?.
Edit
The short null check syntax that I am suggesting would only apply to objects. The short null check would not apply to primitives and types like bool?.
Because if statements in C# are strict. They take only boolean values, nothing else, and there are no subsequent levels of "truthiness" (i.e., 0, null, whatever. They are their own animal and no implicit conversion exists for them).
The compiler could "easily convert" almost any expression to a boolean, but that can cause subtle problems (believe me...) and a conscious decision was made to disallow these implicit conversions.
IMO this was a good choice. You are essentially asking for a one-off implicit conversion where the compiler assumes that, if the expression does not return a boolean result, then the programmer must have wanted to perform a null check. Aside from being a very narrow feature, it is purely syntactic sugar and provides little to no appreciable benefit. As Eric Lippert woudl say, every feature has a cost...
You are asking for a feature which adds needless complexity to the language (yes, it is complex because a type may define an implicit conversion to bool. If that is the case, which check is performed?) only to allow you to not type != null once in a while.
EDIT:
Example of how to define an implicit conversion to bool for #Sam (too long for comments).
class Foo
{
public int SomeVar;
public Foo( int i )
{
SomeVar = i;
}
public static implicit operator bool( Foo f )
{
return f.SomeVar != 0;
}
}
static void Main()
{
var f = new Foo(1);
if( f )
{
Console.Write( "It worked!" );
}
}
One potential collision is with a reference object that defines an implicit conversion to bool.
There is no delineation for the compiler between if(myObject) checking for null or checking for true.
The intent its to leave no ambiguity. You may find it tedious but that short hand is responsible for a number of bugs over the years. C# rightly has a type for booleans and out was a conscience decision not to make 0 mean false and any other value true.
You could write an extension method against System.Object, perhaps called IsNull()?
Of course, that's still an extra 8 or 9 characters on top of the code you'd have to write for the extension class. I think most people are happy with the clarity that an explicit null test brings.
In C#, an anonymous type can be as follows:
method doStuff(){
var myVar = new {
a = false,
b = true
}
if (myVar.a)
{
// Do stuff
}
}
However, the following will not compile:
method doStuff(){
var myVar = new {
a = false,
b = true
}
if (myVar.a)
{
myVar.b = true;
}
}
This is because myVar's fields are read-only and cannot be assigned to. It seems wanting to do something like the latter is fairly common; perhaps the best solution I've seen is to just define a struct outside the method.
However, is there really no other way to make the above block work? The reason it bothers me is, myVar is a local variable of this field, so it seems like it should only be referred to inside the method that uses it. Besides, needing to place the struct outside of the method can make the declaration of an object quite far from its use, especially in a long method.
Put in another way, is there an alternative to anonymous types which will allow me to define a "struct" like this (I realize struct exists in C# and must be defined outside of a method) without making it read-only? If no, is there something fundamentally wrong with wanting to do this, and should I be using a different approach?
No, you'll have to create your own class or struct to do this (preferrably a class if you want it to be mutable - mutable structs are horrible).
If you don't care about Equals/ToString/GetHashCode implementations, that's pretty easy:
public class MyClass {
public bool Foo { get; set; }
public bool Bar { get; set; }
}
(I'd still use properties rather than fields, for various reasons.)
Personally I usually find myself wanting an immutable type which I can pass between methods etc - I want a named version of the existing anonymous type feature...
Is there an alternative to anonymous types which will allow me to concisely define a simple "record" type like this without making it read-only?
No. You'll have to make a nominal type.
If no, is there something fundamentally wrong with wanting to do this?
No, it's a reasonable feature that we have considered before.
I note that in Visual Basic, anonymous types are mutable if you want them to be.
The only thing that is really "fundamentally wrong" about a mutable anonymous type is that it would be dangerous to use one as a hash key. We designed anonymous types with the assumptions that (1) you're going to use them as the keys in equijoins in LINQ query comprehensions, and (2) in LINQ-to-Objects and other implementations, joins will be implemented using hash tables. Therefore anonymous types should be useful as hash keys, and mutable hash keys are dangerous.
In Visual Basic, the GetHashCode implementation does not consume any information from mutable fields of anonymous types. Though that is a reasonable compromise, we simply decided that in C# the extra complexity wasn't worth the effort.
In C# 7 we can leverage named tuples to do the trick:
(bool a, bool b) myVar = (false, true);
if (myVar.a)
{
myVar.b = true;
}
You won't be able to get the nice initialization syntax but the ExpandoObject class introduced in .NET 4 would serve as a viable solution.
dynamic eo = new ExpandoObject();
eo.SomeIntValue = 5;
eo.SomeIntValue = 10; // works fine
For the above types of operation, you should define your own mutable STRUCT. Mutable structs may pose a headache for compiler writers like Eric Lippert, and there are some unfortunate limitations in how .net handles them, but nonetheless the semantics of mutable "Plain Old Data" structs (structs in which all fields are public, and the only public functions which write this are constructors, or are called exclusively from constructors) offer far clearer semantics than can be achieved via classes.
For example, consider the following:
struct Foo {
public int bar;
...other stuff;
}
int test(Action<Foo[]> proc1, Action<Foo> proc2)
{
foo myFoos[] = new Foo[100];
proc1(myFoos);
myFoos[4].bar = 9;
proc2(myFoos[4]); // Pass-by-value
return myFoos[4].bar;
}
Assuming there's no unsafe code and that the passed-in delegates can be called and will return in finite time, what will test() return? The fact that Foo is a struct with a public field bar is sufficient to answer the question: it will return 9, regardless of what else appears in the declaration of Foo, and regardless of what functions are passed in proc1 and proc2. If Foo were a class, one would have to examine every single Action<Foo[]> and Action<Foo> that exists, or will ever exist, to know what test() would return. Determining that Foo is a struct with public field bar seems much easier than examining all past and future functions that might get passed in.
Struct methods which modify this are handled particularly poorly in .net, so if one needs to use a method to modify a struct, it's almost certainly better to use one of these patterns:
myStruct = myStruct.ModifiedInSomeFashion(...); // Approach #1
myStructType.ModifyInSomeFashion(ref myStruct, ...); // Approach #2
than the pattern:
myStruct.ModifyInSomeFashion(...);
Provided one uses the above approach to struct-modifying patterns, however, mutable structs have the advantage of allowing code which is both more efficient and easier to read than immutable structs or immutable classes, and is much less trouble-prone than mutable classes. For things which represent an aggregation of values, with no identity outside the values they contain, mutable class types are often the worst possible representation.
I find it really annoying that you can't set anonymous properties as read/write as you can in VB - often I want to return data from a database using EF/LINQ projection, and then do some massaging of the data in c# that can't be done at the database for whatever reason. The easiest way to do this is to iterate over existing anonymous instances and update properties as you go. NOTE this is not so bad now in EF.Core, as you can mix db functions and .net functions in a single query finally.
My go-to workaround is to use reflection and will be frowned upon and down-voted but works; buyer beware if the underlying implementation changes and all your code breaks.
public static class AnonClassHelper {
public static void SetField<T>(object anonClass, string fieldName, T value) {
var field = anonClass.GetType().GetField($"<{fieldName}>i__Field", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
field.SetValue(anonClass, value);
}
}
// usage
AnonClassHelper.SetField(inst, nameof(inst.SomeField), newVal);
An alternative I have used when dealing with strings is to make properties of type StringBuilder, then these individual properties will be settable via the StringBuilder methods after you have an instance of your anonymous type.
I know it is really old question but how about replacing whole anonymous
object:
`
if (myVar.a)
{
myVar = new
{ a = false, b = true };
}
`
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);
}
}
In functional languages there is often a Maybe monad which allows you to chain multiple calls on an object and have the entire expression return None/null if any part of the chain evaluates to nothing, rather than the typical NullReferenceException you'd get in C# by chaining calls where one object could be null.
This can be trivially implemented by writing a Maybe<T> with some extension methods to allow similar behaviour in C# using query comprehensions, which can be useful when processing XML with optional elements/attributes e.g.
var val = from foo in doc.Elements("foo").FirstOrDefault().ToMaybe()
from bar in foo.Attribute("bar").ToMaybe()
select bar.Value;
But this syntax is a bit clunky and unintuitive as people are used to dealing with sequences in Linq rather than single elements, and it leaves you with a Maybe<T> rather than a T at the end. Would a conditional de-reference operator (e.g. ..) be sufficiently useful to make it into the language? e.g.
var val = doc.Elements("foo").FirstOrDefault()..Attribute("bar")..Value;
The conditional de-reference would expand to something like:
object val;
var foo = doc.Elements("foo").FirstOrDefault();
if (foo != null)
{
var bar = foo.Attribute("bar");
if (bar != null)
{
val = bar.Value;
}
else
{
val = null;
}
}
I can see that this could potentially lead to terrible abuse like using .. everywhere to avoid a NullReferenceException, but on the other hand when used properly it could be very handy in quite a few situations. Thoughts?
Chaining multiple calls on an object makes me fear violations of the Law of Demeter. Thus, I am skeptical that this feature is a good idea, at least in terms of solving the specific problem you are using as an example.
It's an interesting idea that could be achieved with an extension method. Something like this, for example (note, just for example - I'm sure it can be refined):
public static IEnumerable<T> Maybe<T>(this IEnumerable<T> lhs, Func<IEnumerable<T>, T> rhs)
{
if (lhs != null)
{
return rhs(lhs);
}
return lhs;
}
I suspect a combination of NUllable and extension methods would allow a significant portion of this to b achieved.
This would limit T to value types of course.
(TBH I would rather see tuple support in the langauge and eliminate out parameters, as F# does.)
You code can be simplified, set val to null and you eliminate else branches.
I can see the potential usefulness, but other than a slight performance impact if the elements are often null, why not just surround the code block with a try..catch block for a NullReferenceException instead?