As the title suggests I am wondering what the relationship between these two programming concepts are. Does a certain type system (static / dynamic) lend itself to a certain type of scoping (lexical / dynamic), or are these independent language choices?
Static typing doesn't work all that well with dynamic scoping since the variable binding is resolved at runtime. It's possible but it would be unwieldy, since the type system would have to type free variables somehow, probably by examining bound ones. Basically, you couldn't declare two different variables of the same name but different type. Strong and weak typing will also come into play. I'm still pondering what form a static, weakly typed, dynamically scoped language might take, assuming it's possible.
Lexical scoping is paired with both static and dynamic typing.
Related
Is Haskell strongly typed? I.e. is it possible to change the type of a variable after you assigned one? I can't seem to find the answer on the internet.
Static — types are known at compile time. Java and Haskell have static typing. Also C/C++, C#, Go, Scala, Rust, Kotlin, Pascal to list a few more.
A statically typed language might or might not have type inference. Java almost completely lacks type inference (but it's very slowly changing just a little bit); Haskell has full type inference (except with certain very advanced extensions).
(Type inference is when you only have to declare a minimal amount of types by hand, e.g. var isFoo = true and var person = new Person(), instead of bool isFoo = ... and Person person = ....)
Dynamic — Python, JavaScript, Ruby, PHP, Clojure (and Lisps in general), Prolog, Erlang, Groovy etc. Can also be called "unityped"; dynamic typing can be "emulated" in a static setting, but the reverse is not true except by using external static analysis tools. Some languages make it possible to mix dynamic and static (see gradual typing, e.g. https://typedclojure.org/).
Some languages enable static typing for one or more modules, applied at import time, for example: Python+Mypy, Typed Clojure, JavaScript+Flow, PHP+Hack to name a few.
Strong — values that are intended to be treated as Cat always are; trying to treat them like a Dog will cause a loud meeewww... I mean error.
Weak — this effectively boils down to 2 similar but distinct things: type coercion (e.g. "5"+3 equals 8 in PHP — or does it!) and memory reinterpretation (e.g. (int) someCharValue or (bool) somePtr in C, and C++ as well, but C++ wants you to explicitly say reinterpret_cast). So there's really coercion-weak and reinterpretation-weak, and different languages are weak in one or both of these ways.
Interestingly, note that coercion is implicit by nature and memory reinterpretation is explicit (except in Assembly) — so weak typing consists of an implicit and an explicit behavior. Maybe that's even more of a reason to refer to 2 distinct subcategories under weak typing.
There are languages with all 4 possible combinations, and variations/gradations thereof.
Haskell is static+strong; of course it has unsafeCoerce so it can be static+a bit reinterpret-weak at times, but unsafeCoerce is very much frowned upon except in extreme situations where you are sure about something being the case but just can't seem to persuade the compiler without going all the way back and retelling the entire story in a different way.
C is static+weak because all memory can freely be reinterpreted as something it originally was not meant to contain, hence weak. But all of those reinterpretations are kept track of by the type checker, so still fully static too. But C does not do implicit coercions, so it's only reinterpret-weak.
Python is dynamic+almost entirely strong — there are no types known on any given line of code prior to reaching that line during execution, however values that live at runtime do have types associated with them and it's impossible to reinterpret memory. Implicit coercions are also kept to a meaningful minimum, so one might say Python is 99.9% strong and 0.01% coercion-weak.
PHP and JavaScript are dynamic+mostly weak — dynamic, in that nothing has type until you execute and introspect its contents, and also weak in that coercions happen all the time and with things you'd never really expect to be coerced, unless you are only calling methods and functions and not using built-in operations. These coercions are a source of a lot of humor on the internet. There are no memory reinterpretations so PHP and JS are coercion-weak.
Furthermore, some people like to think that static typing is about variables having type, and strong typing is about values having type — this is a very useful way to go about understanding the full picture, but it's not quite true: some dynamically typed languages also allow variables/parameters to be annotated with types/constraints that are enforced at runtime.
In static typing, it's expressions that have a type; the fact of variables having type is only a consequence of variables being used as a means to glue bigger expressions together from smaller ones, so it's not variables per se that have types.
Similarly, in dynamic typing, it's not the variables that lack statically known type — it's all expressions! Variables lacking type is merely a consequence of the expressions they store lacking type.
One final illustration
In dynamic typing, all the cats, dogs and even elephants (in fact entire zoos!) are packaged up in identically sized boxes.
In static typing these boxes look different and have stickers on them saying what's inside.
Some people like it because they can just use a single box form factor and don't have to put any labels on the boxes — it's only the arrangement of boxes with regards to each other that implicitly (and hopefully) provides type sanity.
Some people also like it because it allows them to do all sorts of tricks with tigers temporarily being transported in boxes that smell like lions, and bears put in the same array of interconnected boxes as wolves or deer.
In such label-free setting of transport boxes, all the possible logicistics scenarios need to be played or simulated in order to detect misalignment in the implicit arrangement, like in a stage performance. No reliable guarantees can be given based on reasoning only, generally speaking. (ad-hoc test cases that need for the entire system to be started up for any partial conclusions to be obtained of its soundness)
With labels and explicit rules on how to deal with boxes of various labels, automated/mechanized logical reasoning can be used to draw up conclusions on what the logistics system won't do or will do for sure (static verification, formal proof, or at least pseudo-proof like QuickCheck), Some aspects of the logistics still need to be verified with trial runs, such as whether the logistics team even got the client right. (integration testing, acceptance testing, end user sanity checks).
Moreover, in weak typing dogs can be sliced up and reassembled as frankenstein cats. Whether they like it or not, and whether the result is ugly or not. (weak typing)
But if you add labels to the boxes, it still matters that Frankenstein cats be put in cat boxes. (static+weak typing)
In strong typing, while you can put a cat in the box of a dog, but you can only keep pretending it's a dog until you try to humiliate it by feeding it something only dogs would eat — if that happens, it will scream out loud, but until that time, if you're in dynamic typing, it will silently accept its place (in a static world it would refuse to be put in a dog's box before you can say "kitty").
You seem to mix up dynamic/static and weak/strong typing.
Dynamic or static typing is about whether the type of a variable can be changed during execution.
Weak or strong typing is about being able to predict type errors just from function signatures.
Haskell is both statically and strongly typed.
However, there is no such thing as variable in Haskell so talking about dynamic or static typing makes no sense since every identifier assigned with a value cannot be changed at execution.
EDIT: But like goldenbull said, those typing notions are not clearly defined.
It is strongly typed. See section 2.3 here: Why Haskell matters
I think you are talking about two different things.
First, haskell, and most functional programming (FP) languages, do NOT have the concept "variable". Instead, they use the concept "name" and "value", they just "bind" a value to a name. Once the value is bound, you can not bind another value to the same name, this is the key feature of FP.
Strong typing is another topic. Yes, haskell is strongly typed, and so are most FP languages. Strong typing gives FP the ability of "type inference" which is powerful to eliminate hidden bugs in compile time and help reduce the size of the source code.
Maybe you are comparing haskell with python? Python is also strongly typed. The difference between haskell and python is "static typed" and "dynamic typed". The actual meaning of term "Strong type" and "Weak Type" are ambiguous and fuzzy. That is another long story...
Is there a list of items on the web that you should not use when creating a model or variable?
For example, if I wanted to create apartment listings, naming a model something like Property would be problematic in the future and also confusing since property is a built-in Python function.
I did try Googling this, but couldn't come up with anything.
Thanks!
Rules and constraints about naming depend on the programming language. How an identifier/name is bound depends on the language semantics and its scoping rules: an identifer/name will be bound to different element depending on the scope. Scoping is usally lexical (i.e. static) but some language have dynamic scoping (some variant of lisp).
If names are different, there is no confusion in scoping. If identifiers/names are reused accrossed scopes, an identifier/name might mask another one. This is referred as Shadowing. This is a source of confusion.
Certain reserved names (i.e. keywords) have special meaning. Such keyword can simply be forbidden as names of other elements, or not.
For instance, in Smallatalk self is a keyword. It is still possible to declare a temporary variable self, though. In the scope where the temporary variable is visible, self resolves to the temporary variable, not the usual self that is receiver of the message.
Of course, shadowing can happen between regular names.
Scoping rules take types into consideration as well, and inheritance might introduce shadows.
Another source of confusion related to binding is Method Overloading. In statically typed languages, which method is executed depends on the static types at the call site. In certain cases, overloading makes it confusing to know which method is selected. Both Shadowing and Overloading should avoided to avoid confusions.
If your goal is to translate Python to Javascript, and vice versa, I guess you need to check the scoping rules and keywords of both languages to make sure your translation is not only syntactically correct, but also semantically correct.
Generally, programming languages have 'reserved words' or 'keywords' that you're either not able to use or in some cases are but should stay away from. For Python, you can find that list here.
Most words in most natural languages can have different meanings, according to the context. That's why we use specifiers to make the meaning of a word clear. If in any case you think that some particular identifier may be confusing, you can just add a specifier to make it clear. For example ObjectProperty has probably nothing to do with real estate, even in an application that deals with real estate.
The case you present is no different than using generic identifiers with no attached context. For example a variable named limit or length may have completely different meanings in different programs. Just use identifiers that make sense and document their meaning extensively. Being consistent within your own code base would also be preferable. Do not complicate your life with banned term lists that will never be complete and will only make programming more difficult.
The obvious exceptions are words reserved by your programming language of choice - but then again no decent compiler would allow you to use them anyway...
First-class value can be
passed as an argument
returned from a subroutine
assigned into a variable.
Second-class value just can be passed as an argument.
Third-class value even can't be passed as an argument.
Why should these things defined like that? As I understand, "can be passed as an argument" means it can be pushed into the runtime stack;"can be assigned into a variable" means it can be moved into a different location of the memory; "can be returned from a subroutine" almost has the same meaning of "can be assigned into a variable" since the returned value always be put into a known address, so first class value is totally "movable" or "dynamic",second class value is half "movable" , and third class value is just "static", such as labels in C/C++ which just can be addressed by goto statement, and you can't do nothing with that address except "goto" .Does My understanding make any sense? or what do these three kinds of values mean exactly?
Oh no, I may have to go edit Wikipedia again.
There are really only two distinctions worth making: first-class and not first-class. If Michael Scott talks about a third-class anything, I'll be very depressed.
Ok, so what is "first-class," anyway? Well, it is a term that barely has a technical meaning. The meaning, when present, is usually comparative, and it applies to a thing in a language (I'm being deliberately vague here) that has more privileges than a comparable thing. That's all people mean by it.
Let's look at some examples:
Function pointers in C are first-class values because they can be passed to functions, returned from functions, and stored in heap-allocated data structures just like any other value. Functions in Pascal and Ada are not first-class values because although they can be passed as arguments, they cannot be returned as results or stored in heap-allocated data structures.
Struct types are second-class types in C, because there are no literal expressions of struct type. (Since C99 there are literal initializers with named fields, but this is still not as general as having a literal anywhere you can use an expression.)
Polymorphic values are second-class values in ML because although they can be let-bound to names, they cannot be lambda-bound. Therefore they cannot be passed as arguments. But in Haskell, because Haskell supports higher-rank polymorphism, polymorphic values are first-class. (They can even be stored in data structures!)
In Java, the type int is second class because you can't inherit from it. Type Integer is first class.
In C, labels are second class, because they don't have values and you can't compute with them. In FORTRAN, line numbers have values and so are first class. There is a GNU extension to C that allows you to define first-class labels, and it is jolly useful. What does first-class mean in this case? It means the labels have values, can be stored in data structures, and can be used in goto. But those values are second class in another sense, because a label from one procedure can't meaningfully be used in a goto that belongs to another procedure.
Are we getting an idea how useless this terminology is?
I hope these examples convince you that the idea of "first-class" is not a very useful idea in thinking about programming languages overall. When you're talking about a particular feature of a particular language or language family, it can be a useful shorthand ("a language isn't functional unless it has first-class, nested functions") but by and large you're better off saying just what you mean instead of talking about "first-class" or "not first-class" things.
As for "third class", just say no.
Something is first-class if it is explicitly manipulable in the code. In other words, something is first-class if it can be programmatically manipulated at run-time.
This closely relates to meta-programming in the sense that what you describe in the code (at development time) is one meta-level, and what exists at run-time is another meta-level. But the barrier between these two meta-levels can be blurred, for instance with reflection. When something is reified at run-time, it becomes explicitly manipulable.
We speak of first-class object, because objects can be manipulated programmatically at run-time (that's the very purpose).
In java, you have classes, but they are not first-class, because the code can normally not manipulate a class unless you use reflection. But in Smalltalk, classes are first-class: the code can manipulate a class like an regular object.
In java, you have packages (modules), but they are not first-class, because the code does not manipulate package at run-time. But in NewSpeak, packages (modules) are first-class, you can instantiate a module and pass it to another module to specify the modularity at run-time.
In C#, you have closures which are first-class functions. They exist and can be manipulated at run-time programmatically. Such things does not exists (yet) in java.
To me, the boundary first-class/not first-class is not exactly strict. It is sometimes hard to pronounce for some language constructs, e.g. java primitive types. We could say it's not first-class because it's not an object and is not manipulable through a reference that can be passed along, but the primitive value does still exists and can be manipulated at run-time.
PS: I agree with Norman Ramsey and 2nd-class and 3rd-class value make no sense to me.
First-class: A first-class construct is one which is an intrinsic element of a language. The following properties must hold.
It must form part of the lexical syntax of the language
It may have operators applied to it
It must be referenceable (for example stored in a variable)
Second-class: A second-class construct is one which is an intrinsic element of the language with the following properties.
It must form part of the lexical syntax of the language
It may have operators applied to it
Third-class: A third-class construct is one which forms part of the syntax of a language.
in
Roger Keays and Andry Rakotonirainy. Context-oriented programming. In Pro- ceedings of the 3rd ACM International Workshop on Data Engineering for Wire- less and Mobile Access, MobiDe ’03, pages 9–16, New York, NY, USA, 2003. ACM.
Those terms are very broad and not really globally well defined, but here are the most logical definitions for them:
First-class values are the ones that have actual, tangible values, and so can be operated on and go around, as variables, arguments, return values or whatever.
This doesn't really need a thorough example, does it? In C, an int is first-class.
Second-class values are more limited. They have values, but they can't be used directly, so the compiler deliberately limits what you can do with it. You can reference them, so you can still have a first-class value representing them.
For example, in C, a function is a second-class value. It can't be altered, but it can be called and referenced.
Third-class values are even more limited. They not only don't have values, but interaction is completely absent, and often it only exists to be used as compile-time attributes.
For example, in Rust, a lifetime is a third-class value. You can't use the lifetime at all. You can only receive it as a template parameter, you can only use it as a template parameter (only when creating a new variable), and that's all you can do with it.
Another example, in C++, a struct or a class is a third-class value. This doesn't need much explanation.
Suppose you have this pseudo-code
do_something();
function do_something(){
print "I am saying hello.";
}
Why do some programming languages require the call to do_something() to appear below the function declaration in order for the code to run?
Programming languages use a symbol table to hold the various classes, functions, etc. that are used in the source code. Some languages compile in a single pass, whereby the symbols are pulled out of the symbol table as soon as they are used. Others use two passes, where the first pass is used to populate the table, and then the second is used to find the entries.
Most languages with a static type system are designed to require definition before use, which means there must be some sort of declaration of a function before the call so that the call can be checked (e.g., is the function getting the right number and types of arguments). This sort of design helps both a person and a compiler reading the program: everything you see has already been defined. The ease of reading and the popularity of one-pass compilers may explain the popularity of this design rule.
Unfortunately definition before use does not play well with mutual recursion, and so language designers resorted to an ugly hack whereby you have
Declaration (sometimes called a "forward declaration" from the keyword in Pascal)
Use
Definition
You see the same phenomenon at the type level in C in the form of the "incomplete struct declaration."
Around 1990 some language designers figured out that the one-pass compiler with no abstract-syntax tree should be a thing of the past, and two very nice designs from that era—Modula-3 and Haskell got rid of definition before use: in those languages, any defined function or variable is visible throughout its scope, including parts of the program textually before the definition. In other words, mutual recursion is the default for both types and functions. Good on them, I say—these languages have no ugly and unnecessary forward declarations.
Why [have definition before use]?
Easy to write a one-pass compiler in 1975.
without definition before use, you have to think harder about mutual recursion, especially mutually recursive type definitions.
Some people think it makes it easier for a person to read the code.
If I design a new language with type inference, no explicit types and no class inheritance support and then want to add inheritance, what are the minimum extra hints to the compiler needed to resolve type ambiguity when adding the feature?
Are class names needed?
EDIT
The type-tainting is traced through assignments throughout the program including between functions.
OK in the starting language, you have class names like MyTypeName1 from stuff like:
myVariable1 = New(MyTypeName1)
myVariable2 = New(MyTypeName2)
And types MyTypeName1 and MyTypeName2 are inferred for myVariable1 and myVariable2 respectively. But then if we want to enhance the language to support:
MyVariable3 = myVariable1
MyVariable3 = myVariable2
which can be traced through the code (myVariable3 now can contain two types which presumably are in a hierarchy).
EDIT
The members of MyTypeName1 and MyTypeName2 are inferred from statements like:
myVariable1.name="Fred"
myVariable2.name="JX3009"
What if the name member in MyTypeName1 and in MyTypeName2 are not to be in a common base class? What if we want a different name property in MyTypeName1 and MyTypeName2 and none in a base class? Is there an elegant way to tell the compiler what to do / how to handle? (The idea with type inference is to reduce typing not increase it...?)
Or do we need to specify the hierarchy explicitly?
This field is loaded with undecidability results, but I've forgotten them all. If you're willing to conflate inheritance with subtyping, then you have no problems. If you want something more ambitious (and it sounds as if you do), I'd have a look at
Benjamin Pierce's textbook Types and Programming Languages, which will cover the basic results for width and depth subtyping and how they do or do not play nicely with type inference. (I'd look myself but my copy is at work.)
François Pottier's PhD dissertation.
Another name to check out is Joe Wells, who's been responsible for a lot of undecidability results in type systems and programming languages.
What are the minimum extra hints to the compiler needed to resolve type ambiguity?
If there is a unique answer to this question, I will be very surprised. I think it far more likely that this is one of those language-design problems where there are a bunch of solutions which are strictly incomparable as far as minimality and annotations go.