I started using smalltalk and I am trying to add a new assignment operator :>.
The current operator used in pharo is no selector so I started to look into the class Scanner where the underscore _ can be enabled for assignments. I've tried to do it in a similar way but it did not work.
Do you have any idea or suggestion about how I achieve that?
For a start, have a look at the Method Scanner>>#xColon. This method is called whenever a colon is encountered in the input. By adding the following snippet to the top of the method, you can make it detect your new assignment token :>:
aheadChar = $> ifTrue: [
self step.
tokenType := #leftArrow.
self step.
^ token := #':=' ]
Like this :> behaves exactly like the normal assignment. By customising tokenType and token you can pass your new assignment operator to the parser (see Parser>>#expression and Parser>>#assignment:) and build a different AST (i.e. to achieve a different execution behaviour).
If you are interested in more flexibility you might want to look at Helvetia. Helvetia is a language workbench for Pharo that allows you to modify the host language and adapt the tools in a more modular fashion. One of the examples included with the distribution (CUSwapExample) is adding two new assignment operators to Smalltalk.
Related
I am using firebase authentication and adding a listener for authentication state changes as:
var handle = auth?.addStateDidChangeListener(self.updateUI(auth:user:))
while updateUI is a function I have created with signature: (Auth, User?) -> void
I don't understand the syntax of "(auth:user:)" and was thinking perhaps I need a "," in between auth and user, but that gives me compiler error. I'd appreciate if someone can explain this to me
By writing updateUI(auth:user:), what you are referring to is the method itself, and you are not calling the method immediately. This is an explicit-member-expression as the language reference calls it. And the language reference says that one of the forms that an explicit-member-expression can take is:
As you can see from the formal grammar, inside the parentheses, there can be zero or more argument-name, and an argument-name is an identifier followed by the character :.
So why don't you need ,?
Because the language reference says so. :)
If you think about it, the : is already delimiting the different parameter labels, so you don't need an extra delimiter.
Why write out the parameter labels in the first place?
It is likely to avoid ambiguity. There's probably another overload of updateUI with different parameter labels, so just saying updateUI could be ambiguous. If there is only one updateUI, then you can just say updateUI.
What is the difference between = and := in Scala?
I have googled extensively for "scala colon-equals", but was unable to find anything definitive.
= in scala is the actual assignment operator -- it does a handful of specific things that for the most part you don't have control over, such as
Giving a val or var a value when it's created
Changing the value of a var
Changing the value of a field on a class
Making a type alias
Probably others
:= is not a built-in operator -- anyone can overload it and define it to mean whatever they like. The reason people like to use := is because it looks very assignmenty and is used as an assignment operator in other languages.
So, if you're trying to find out what := means in the particular library you're using... my advice is look through the Scaladocs (if they exist) for a method named :=.
from Martin Odersky:
Initially we had colon-equals for assignment—just as in Pascal, Modula, and Ada—and a single equals sign for equality. A lot of programming theorists would argue that that's the right way to do it. Assignment is not equality, and you should therefore use a different symbol for assignment. But then I tried it out with some people coming from Java. The reaction I got was, "Well, this looks like an interesting language. But why do you write colon-equals? What is it?" And I explained that its like that in Pascal. They said, "Now I understand, but I don't understand why you insist on doing that." Then I realized this is not something we wanted to insist on. We didn't want to say, "We have a better language because we write colon-equals instead of equals for assignment." It's a totally minor point, and people can get used to either approach. So we decided to not fight convention in these minor things, when there were other places where we did want to make a difference.
from The Goals of Scala's Design
= performs assignment. := is not defined in the standard library or the language specification. It's a name that is free for other libraries or your code to use, if you wish.
Scala allows for operator overloading, where you can define the behaviour of an operator just like you could write a method.
As in other languages, = is an assignment operator.
The is no standard operator I'm aware of called :=, but could define one with this name. If you see an operator like this, you should check up the documentation of whatever you're looking at, or search for where that operator is defined.
There is a lot you can do with Scala operators. You can essentially make an operator out of virtually any characters you like.
I'm new to StringTemplate4 and probably I am going to ask something overly simple, impossible or stupid but I couldn't find any other information on it.
So far, I have set this minimal set of templates:
define(name,arity) ::= "<name>(<vars(arity)>)."
vars(n) ::= "<n:var();separator=\", \">"
var(n) ::= "V<n>"
and I would like to get:
pred(V1, V2, V3).
by calling the following code:
STGroup group = new STGroupFile(...);
ST st = group.getInstanceOf("define");
st.add("name", "pred");
st.add("arity", 3);
String result = st.render();
Is it possible? Many thanks in advance.
StringTemplate doesn't have built-in operators for repeating. Instead, you'll need to iterate, like described in the following question.
Is there anything like Enumerable.Range(x,y) in Java?
Keep in mind that you'll need to pass an Iterator<T> and not an Iterable<T> due to a current limitation in StringTemplate (the interpreter supports Collection<T>, but not Iterable<T>). If you want to use the built-in iteration variable i or i0, you could also pass an appropriately sized new Object[n].
What is the difference between = and := in Scala?
I have googled extensively for "scala colon-equals", but was unable to find anything definitive.
= in scala is the actual assignment operator -- it does a handful of specific things that for the most part you don't have control over, such as
Giving a val or var a value when it's created
Changing the value of a var
Changing the value of a field on a class
Making a type alias
Probably others
:= is not a built-in operator -- anyone can overload it and define it to mean whatever they like. The reason people like to use := is because it looks very assignmenty and is used as an assignment operator in other languages.
So, if you're trying to find out what := means in the particular library you're using... my advice is look through the Scaladocs (if they exist) for a method named :=.
from Martin Odersky:
Initially we had colon-equals for assignment—just as in Pascal, Modula, and Ada—and a single equals sign for equality. A lot of programming theorists would argue that that's the right way to do it. Assignment is not equality, and you should therefore use a different symbol for assignment. But then I tried it out with some people coming from Java. The reaction I got was, "Well, this looks like an interesting language. But why do you write colon-equals? What is it?" And I explained that its like that in Pascal. They said, "Now I understand, but I don't understand why you insist on doing that." Then I realized this is not something we wanted to insist on. We didn't want to say, "We have a better language because we write colon-equals instead of equals for assignment." It's a totally minor point, and people can get used to either approach. So we decided to not fight convention in these minor things, when there were other places where we did want to make a difference.
from The Goals of Scala's Design
= performs assignment. := is not defined in the standard library or the language specification. It's a name that is free for other libraries or your code to use, if you wish.
Scala allows for operator overloading, where you can define the behaviour of an operator just like you could write a method.
As in other languages, = is an assignment operator.
The is no standard operator I'm aware of called :=, but could define one with this name. If you see an operator like this, you should check up the documentation of whatever you're looking at, or search for where that operator is defined.
There is a lot you can do with Scala operators. You can essentially make an operator out of virtually any characters you like.
I'm currently experimenting with using OCaml and GTK together (using the lablgtk bindings). However, the documentation isn't the best, and while I can work out how to use most of the features, I'm stuck with changing notebook pages (switching to a different tab).
I have found the function that I need to use, but I don't know how to use it. The documentation seems to suggest that it is in a sub-module of GtkPackProps.Notebook, but I don't know how to call this.
Also, this function has a type signature different to any I have seen before.
val switch_page : ([> `notebook ], Gpointer.boxed option -> int -> unit) GtkSignal.t
I think it returns a GtkSignal.t, but I have no idea how to pass the first parameter to the function (the whole part in brackets).
Has anyone got some sample code showing how to change the notebook page, or can perhaps give me some tips on how to do this?
What you have found is not a function but the signal. The functional type you see in its type is the type of the callback that will be called when the page switch happen, but won't cause it.
by the way the type of switch_page is read as: a signal (GtkSignal.t) raised by notebook [> `notebook ], whose callbacks have type Gpointer.boxed option -> int -> unit
Generally speaking, with lablgtk, you'd better stay away of the Gtk* low level modules, and use tge G[A-Z] higher level module. Those module API look like the C Gtk one, and I always use the main Gtk doc to help myself.
In your case you want to use the GPack.notebook object and its goto_page method.
You've found a polymorphic variant; they're described in the manual in Section 4.2, and the typing rules always break my head. I believe what the signature says is that the function switch_page expects as argument a GtkSignal.t, which is an abstraction parameterized by two types:
The first type parameter,
[> `notebook]
includes as values any polymorphic variant including notebook (that's what the greater-than means).
The second type parameter is an ordinary function.
If I'm reading the documentation for GtkSignal.t correctly, it's not a function at all; it's a record with three fields:
name is a string.
classe is a polymorphic variant which could be ``notebook` or something else.
marshaller is a marshaller for the function type Gpointer.boxed option -> int -> unit.
I hope this helps. If you have more trouble, section 4.2 of the manual, on polymorphic variants, might sort you out.