What is the difference between statically typed and dynamically typed languages

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I hear a lot that new programming languages are dynamically typed but what does it actually mean when we say a language is dynamically typed vs  statically typed

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Statically typed languages type-check at compile time and the type can NOT change. (Don't get cute with type-casting comments, a new variable/reference is created).

Dynamically typed languages type-check at run-time and the type of an variable CAN be changed at run-time.

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Static Typing   The languages such as Java and Scala are static typed   The variables have to be defined and initialized before they are used in a code   for ex  int x  x   10   System out println x    Dynamic Typing   Perl is an dynamic typed language   Variables need not be initialized before they are used in code   y 10  use this variable in the later part of code

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Statically Typed  The types are checked before run-time so mistakes can be caught earlier   Examples   c    Dynamically Typed  The types are checked during execution   Examples   Python

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Here is an example contrasting how Python  dynamically typed  and Go  statically typed  handle a type error    def silly a       if a  gt  0          print  Hi      else          print 5    3    Python does type checking at run time  and therefore   silly 2    Runs perfectly fine  and produces the expected output Hi  Error is only raised if the problematic line is hit   silly -1    Produces   TypeError  unsupported operand type s  for     int  and  str     because the relevant line was actually executed   Go on the other hand does type-checking at compile time   package main  import   fmt     func silly a int        if  a  gt  0            fmt Println  Hi         else           fmt Println  3    5           func main         silly 2      The above will not compile  with the following error    invalid operation   3    5  mismatched types string and int

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The terminology  dynamically typed  is unfortunately misleading  All languages are statically typed  and types are properties of expressions  not of values as some think   However  some languages have only one type  These are called uni-typed languages  One example of such a language is the untyped lambda calculus   In the untyped lambda calculus  all terms are lambda terms  and the only operation that can be performed on a term is applying it to another term  Hence all operations always result in either infinite recursion or a lambda term  but never signal an error   However  were we to augment the untyped lambda calculus with primitive numbers and arithmetic operations  then we could perform nonsensical operations  such adding two lambda terms together    x x      y y   One could argue that the only sane thing to do is to signal an error when this happens  but to be able to do this  each value has to be tagged with an indicator that indicates whether the term is a lambda term or a number  The addition operator will then check that indeed both arguments are tagged as numbers  and if they aren t  signal an error  Note that these tags are not types  because types are properties of programs  not of values produced by those programs   A uni-typed language that does this is called dynamically typed   Languages such as JavaScript  Python  and Ruby are all uni-typed  Again  the typeof operator in JavaScript and the type function in Python have misleading names  they return the tags associated with the operands  not their types  Similarly  dynamic cast in C   and instanceof in Java do not do type checks

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Statically typed languages like C    Java and Dynamically typed languages like Python differ only in terms of the execution of the type of the variable     Statically typed languages have static data type for the variable  here the data type is checked during compiling so debugging is much simpler   whereas Dynamically typed languages don t do the same  the data type is checked which executing the program and hence the debugging is bit difficult   Moreover they have a very small difference and can be related with strongly typed and weakly typed languages  A strongly typed language doesn t allow you to use one type as another eg  C and C      whereas weakly typed languages allow eg python

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dynamically typed language helps to quickly prototype algorithm concepts without the overhead of about thinking what variable types need to be used  which is a necessity in statically typed language

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Statically typed languages  A language is statically typed if the type of a variable is known at compile time  For some languages this means that you as the programmer must specify what type each variable is  e g   Java  C  C     other languages offer some form of type inference  the capability of the type system to deduce the type of a variable  e g   OCaml  Haskell  Scala  Kotlin   The main advantage here is that all kinds of checking can be done by the compiler  and therefore a lot of trivial bugs are caught at a very early stage   Examples  C  C    Java  Rust  Go  Scala  Dynamically typed languages  A language is dynamically typed if the type is associated with run-time values  and not named variables fields etc  This means that you as a programmer can write a little quicker because you do not have to specify types every time  unless using a statically-typed language with type inference     Examples  Perl  Ruby  Python  PHP  JavaScript  Most scripting languages have this feature as there is no compiler to do static type-checking anyway  but you may find yourself searching for a bug that is due to the interpreter misinterpreting the type of a variable  Luckily  scripts tend to be small so bugs have not so many places to hide   Most dynamically typed languages do allow you to provide type information  but do not require it  One language that is currently being developed  Rascal  takes a hybrid approach allowing dynamic typing within functions but enforcing static typing for the function signature

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Simply put it this way  in a statically typed language variables  types are static  meaning once you set a variable to a type  you cannot change it  That is because typing is associated with the variable rather than the value it refers to    For example in Java    String str    Hello      variable str statically typed as string str   5                  would throw an error since str is supposed to be a string only   Where on the other hand  in a dynamically typed language variables  types are dynamic  meaning after you set a variable to a type  you CAN change it  That is because typing is associated with the value it assumes rather than the variable itself    For example in Python   str    Hello    variable str is linked to a string value str   5         now it is linked to an integer value  perfectly OK   So  it is best to think of variables in dynamically typed languages as  just generic pointers  to typed values    To sum up  type describes  or should have described  the variables in the language rather than the language itself  It could have been better used as a language with statically typed variables versus a language with dynamically typed variables IMHO   Statically typed languages are generally compiled languages  thus  the compilers check the types  make perfect sense right  as types are not allowed to be changed later on at run time    Dynamically typed languages are generally interpreted  thus type checking  if any  happens at run time when they are used  This of course brings some performance cost  and is one of the reasons dynamic languages  e g   python  ruby  php  do not scale as good as the typed ones  java  c   etc    From another perspective  statically typed languages have more of a start-up cost  makes you usually write more code  harder code  But that pays later off    The good thing is both sides are borrowing features from the other side  Typed languages are incorporating more dynamic features  e g   generics and dynamic libraries in c   and dynamic languages are including more type checking  e g   type annotations in python  or HACK variant of PHP  which are usually not core to the language and usable on demand    When it comes to technology selection  neither side has an intrinsic superiority over the other  It is just a matter of preference whether you want more control to begin with or flexibility  just pick the right tool for the job  and make sure to check what is available in terms of the opposite before considering a switch

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In a statically typed language  a variable is associated with a type which is known at compile time  and that type remains unchanged throughout the execution of a program  Equivalently  the variable can only be assigned a value which is an instance of the known specified type  In a dynamically typed language  a variable has no type  and its value during execution can be anything of any shape and form

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Compiled vs  Interpreted   When source code is translated    Source Code  Original code  usually typed by a human into a computer  Translation  Converting source code into something a computer can read  i e  machine code  Run-Time  Period when program is executing commands  after compilation  if compiled  Compiled Language  Code translated before run-time Interpreted Language  Code translated on the fly  during execution     Typing   When types are checked   5    3  is an example of a type error in strongly typed languages such as Go and Python  because they don t allow for  type coercion  -  the ability for a value to change type in certain contexts such as merging two types  Weakly typed languages  such as JavaScript  won t throw a type error  results in  53      Static  Types checked before run-time Dynamic  Types checked on the fly  during execution     The definitions of  Static  amp  Compiled  and  Dynamic  amp  Interpreted  are quite similar   but remember it s  when types are checked  vs   when source code is translated     You ll get the same type errors irrespective of whether the language is compiled or interpreted  You need to separate these terms conceptually     Python Example  Dynamic  Interpreted  def silly a       if a  gt  0          print  Hi      else          print 5    3   silly 2    Because Python is both interpreted and dynamically typed  it only translates and type-checks code it s executing on  The else block never executes  so 5    3  is never even looked at   What if it was statically typed   A type error would be thrown before the code is even run  It still performs type-checking before run-time even though it is interpreted   What if it was compiled    The else block would be translated looked at before run-time  but because it s dynamically typed it wouldn t throw an error  Dynamically typed languages don t check types until execution  and that line never executes     Go Example  Static  Compiled  package main  import   fmt     func silly a int      if  a  gt  0          fmt Println  Hi       else         fmt Println  3    5         func main       silly 2      The types are checked before running  static  and the type error is immediately caught  The types would still be checked before run-time if it was interpreted  having the same result  If it was dynamic  it wouldn t throw any errors even though the code would be looked at during compilation     Performance  A compiled language will have better performance at run-time if it s statically typed  vs  dynamically   knowledge of types allows for machine code optimization   Statically typed languages have better performance at run-time intrinsically due to not needing to check types dynamically while executing  it checks before running    Similarly  compiled languages are faster at run time as the code has already been translated instead of needing to  interpret  translate it on the fly   Note that both compiled and statically typed languages will have a delay before running for translation and type-checking  respectively     More Differences  Static typing catches errors early  instead of finding them during execution  especially useful for long programs   It s more  strict  in that it won t allow for type errors anywhere in your program and often prevents variables from changing types  which further defends against unintended errors   num   2 num    3     ERROR   Dynamic typing is more flexible  which some appreciate  It typically allows for variables to change types  which can result in unexpected errors

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http   en wikipedia org wiki Type system  Static typing A programming language is said to use static typing when type checking is performed during compile-time as opposed to run-time  In static typing  types are associated with variables not values  Statically typed languages include Ada  C  C    C   JADE  Java  Fortran  Haskell  ML  Pascal  Perl  with respect to distinguishing scalars  arrays  hashes and subroutines  and Scala  Static typing is a limited form of program verification  see type safety   accordingly  it allows many type errors to be caught early in the development cycle  Static type checkers evaluate only the type information that can be determined at compile time  but are able to verify that the checked conditions hold for all possible executions of the program  which eliminates the need to repeat type checks every time the program is executed  Program execution may also be made more efficient  i e  faster or taking reduced memory  by omitting runtime type checks and enabling other optimizations  Because they evaluate type information during compilation  and therefore lack type information that is only available at run-time  static type checkers are conservative  They will reject some programs that may be well-behaved at run-time  but that cannot be statically determined to be well-typed  For example  even if an expression  always evaluates to true at run-time  a program containing the code if  lt complex test gt  then 42 else  lt type error gt   will be rejected as ill-typed  because a static analysis cannot determine that the else branch won t be taken  1  The conservative behaviour of static type checkers is advantageous when  evaluates to false infrequently  A static type checker can detect type errors in rarely used code paths  Without static type checking  even code coverage tests with 100  code coverage may be unable to find such type errors  Code coverage tests may fail to detect such type errors because the combination of all places where values are created and all places where a certain value is used must be taken into account  The most widely used statically typed languages are not formally type safe  They have  quot loopholes quot  in the programming language specification enabling programmers to write code that circumvents the verification performed by a static type checker and so address a wider range of problems  For example  Java and most C-style languages have type punning  and Haskell has such features as unsafePerformIO  such operations may be unsafe at runtime  in that they can cause unwanted behaviour due to incorrect typing of values when the program runs  Dynamic typing A programming language is said to be dynamically typed  or just  dynamic   when the majority of its type checking is performed at run-time as opposed to at compile-time  In dynamic typing  types are associated with values not variables  Dynamically typed languages include Groovy  JavaScript  Lisp  Lua  Objective-C  Perl  with respect to user-defined types but not built-in types   PHP  Prolog  Python  Ruby  Smalltalk and Tcl  Compared to static typing  dynamic typing can be more flexible  e g  by allowing programs to generate types and functionality based on run-time data   though at the expense of fewer a priori guarantees  This is because a dynamically typed language accepts and attempts to execute some programs which may be ruled as invalid by a static type checker  Dynamic typing may result in runtime type errors   that is  at runtime  a value may have an unexpected type  and an operation nonsensical for that type is applied  This operation may occur long after the place where the programming mistake was made   that is  the place where the wrong type of data passed into a place it should not have  This makes the bug difficult to locate  Dynamically typed language systems  compared to their statically typed cousins  make fewer  quot compile-time quot  checks on the source code  but will check  for example  that the program is syntactically correct   Run-time checks can potentially be more sophisticated  since they can use dynamic information as well as any information that was present during compilation  On the other hand  runtime checks only assert that conditions hold in a particular execution of the program  and these checks are repeated for every execution of the program  Development in dynamically typed languages is often supported by programming practices such as unit testing  Testing is a key practice in professional software development  and is particularly important in dynamically typed languages  In practice  the testing done to ensure correct program operation can detect a much wider range of errors than static type-checking  but conversely cannot search as comprehensively for the errors that both testing and static type checking are able to detect  Testing can be incorporated into the software build cycle  in which case it can be thought of as a  quot compile-time quot  check  in that the program user will not have to manually run such tests  References  Pierce  Benjamin  2002   Types and Programming Languages  MIT Press  ISBN 0-262-16209-1

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Statically typed languages  each variable and expression is already known at compile time     int a  a can take only integer type values at runtime   Examples  C  C    Java  Dynamically typed languages  variables can receive different values at runtime and their type is defined at run time     var a  a can take any kind of values at runtime   Examples  Ruby  Python

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Statically typed programming languages do type checking  i e  the process of verifying and enforcing the constraints of types  at compile-time as opposed to run-time   Dynamically typed programming languages do type checking at run-time as opposed to compile-time   Examples of statically typed languages are  -  Java  C  C    Examples of dynamically typed languages are  - Perl  Ruby  Python  PHP  JavaScript

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Sweet and simple definitions  but fitting the need  Statically typed languages binds the type to a variable for its entire scope  Seg  SCALA  Dynamically typed languages bind the type to the actual value referenced by a variable

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Static typed languages  compiler resolves method calls and compile references     usually better performance faster compile error feedback better IDE support not suited for working with undefined data formats harder to start a development when model is not defined when  longer compilation time in many cases requires to write more code   Dynamic typed languages  decisions taken in running program     lower performance faster development some bugs might be detected only later in run-time good for undefined data formats  meta programming

[programming-languages] What is the difference between statically typed and dynamically typed languages?

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