What is declarative programming

Question

I keep hearing this term tossed around in several different contexts  What is it

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It may sound odd  but I d add Excel  or any spreadsheet really  to the list of declarative systems   A good example of this is given here

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I have refined my understanding of declarative programming  since Dec 2011 when I provided an answer to this question  Here follows my current understanding   The long version of my understanding  research  is detailed at this link  which you should read to gain a deep understanding of the summary I will provide below   Imperative programming is where mutable state is stored and read  thus the ordering and or duplication of program instructions can alter the behavior  semantics  of the program  and even cause a bug  i e  unintended behavior    In the most naive and extreme sense  which I asserted in my prior answer   declarative programming  DP  is avoiding all stored mutable state  thus the ordering and or duplication of program instructions can NOT alter the behavior  semantics  of the program   However  such an extreme definition would not be very useful in the real world  since nearly every program involves stored mutable state  The spreadsheet example conforms to this extreme definition of DP  because the entire program code is run to completion with one static copy of the input state  before the new states are stored  Then if any state is changed  this is repeated  But most real world programs can t be limited to such a monolithic model of state changes   A more useful definition of DP is that the ordering and or duplication of programming instructions do not alter any opaque semantics  In other words  there are not hidden random changes in semantics occurring-- any changes in program instruction order and or duplication cause only intended and transparent changes to the program s behavior   The next step would be to talk about which programming models or paradigms aid in DP  but that is not the question here

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Here s an example    In CSS  used to style HTML pages   if you want an image element to be 100 pixels high and 100 pixels wide  you simply  declare  that that s what you want as follows    myImageId   height  100px  width  100px      You can consider CSS a declarative  style sheet  language   The browser engine that reads and interprets this CSS is free to make the image appear this tall and this wide however it wants  Different browser engines  e g   the engine for IE  the engine for Chrome  will implement this  task differently    Their unique implementations are  of course  NOT written in a declarative language but in a procedural one like Assembly  C  C    Java  JavaScript  or Python  That code is a bunch of steps to be carried out step by step  and might include function calls   It might do things like interpolate pixel values  and render on the screen

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Describing to a computer what you want  not how to do something

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Declarative programming is  the act of programming in languages that conform to the mental model of the developer rather than the operational model of the machine    The difference between declarative and imperative programming is well  illustrated by the problem of parsing structured data   An imperative program would use mutually recursive functions to consume input  and generate data  A declarative program would express a grammar that defines  the structure of the data so that it can then be parsed   The difference between these two approaches is that the declarative program  creates a new language that is more closely mapped to the mental model of the  problem than is its host language

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A couple other examples of declarative programming    ASP Net markup for databinding   It just says  fill this grid with this source   for example  and leaves it to the system for how that happens  Linq expressions   Declarative programming is nice because it can help simplify your mental model  of code  and because it might eventually be more scalable   For example  let s say you have a function that does something to each element in an array or list   Traditional code would look like this   foreach  object item in MyList       DoSomething item       No big deal there   But what if you use the more-declarative syntax and instead define DoSomething   as an Action   Then you can say it this way   MyList ForEach DoSometing     This is  of course  more concise   But I m sure you have more concerns than just saving two lines of code here and there   Performance  for example   The old way  processing had to be done in sequence   What if the  ForEach   method had a way for you to signal that it could handle the processing in parallel  automatically   Now all of a sudden you ve made your code multi-threaded in a very safe way and only changed one line of code   And  in fact  there s a an extension for  Net that lets you do just that    If you follow that link  it takes you to a blog post by a friend of mine   The whole post is a little long  but you can scroll down to the heading titled  The Problem   and pick it up there no problem

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The other answers already do a fantastic job explaining what declarative programming is  so I m just going to provide some examples of why that might be useful   Context Independence  Declarative Programs are context-independent  Because they only declare what the ultimate goal is  but not the intermediary steps to reach that goal  the same program can be used in different contexts  This is hard to do with imperative programs  because they often depend on the context  e g  hidden state    Take yacc as an example  It s a parser generator aka  compiler compiler  an external declarative DSL for describing the grammar of a language  so that a parser for that language can automatically be generated from the description  Because of its context independence  you can do many different things with such a grammar     Generate a C parser for that grammar  the original use case for yacc  Generate a C   parser for that grammar Generate a Java parser for that grammar  using Jay  Generate a C  parser for that grammar  using GPPG  Generate a Ruby parser for that grammar  using Racc  Generate a tree visualization for that grammar  using GraphViz  simply do some pretty-printing  fancy-formatting and syntax highlighting of the yacc source file itself and include it in your Reference Manual as a syntactic specification of your language   And many more  hellip   Optimization  Because you don t prescribe the computer which steps to take and in what order  it can rearrange your program much more freely  maybe even execute some tasks in parallel  A good example is a query planner and query optimizer for a SQL database  Most SQL databases allow you to display the query that they are actually executing vs  the query that you asked them to execute  Often  those queries look nothing like each other  The query planner takes things into account that you wouldn t even have dreamed of  rotational latency of the disk platter  for example or the fact that some completely different application for a completely different user just executed a similar query and the table that you are joining with and that you worked so hard to avoid loading is already in memory anyway   There is an interesting trade-off here  the machine has to work harder to figure out how to do something than it would in an imperative language  but when it does figure it out  it has much more freedom and much more information for the optimization stage

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It depends on how you submit the answer to the text  Overall you can look at the programme at a certain view but it depends what angle you look at the problem  I will get you started with the programme  Dim Bus  Car  Time  Height As Integr  Again it depends on what the problem is an overall  You might have to shorten it due to the programme  Hope this helps and need the feedback if it does not  Thank You

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As far as I can tell  it started being used to describe programming systems like Prolog  because prolog is  supposedly  about declaring things in an abstract way   It increasingly means very little  as it has the definition given by the users above  It should be clear that there is a gulf between the declarative programming of Haskell  as against the declarative programming of HTML

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Since I wrote my prior answer  I have formulated a new definition of the declarative property which is quoted below  I have also defined imperative programming as the dual property   This definition is superior to the one I provided in my prior answer  because it is succinct and it is more general  But it may be more difficult to grok  because the implication of the incompleteness theorems applicable to programming and life in general are difficult for humans to wrap their mind around   The quoted explanation of the definition discusses the role pure functional programming plays in declarative programming      Declarative vs  Imperative      The declarative property is weird  obtuse  and difficult to capture in a technically precise definition that remains general and not ambiguous  because it is a naive notion that we can declare the meaning  a k a semantics  of the program without incurring unintended side effects  There is an inherent tension between expression of meaning and avoidance of unintended effects  and this tension actually derives from the incompleteness theorems of programming and our universe       It is oversimplification  technically imprecise  and often ambiguous to define declarative as    what nbsp to nbsp do    and imperative as    how nbsp to nbsp do     An ambiguous case is the    what    is the    how    in a program that outputs a program    a compiler       Evidently the unbounded recursion that makes a language Turing complete  is also analogously in the semantics    not only in the syntactical structure of evaluation  a k a  operational semantics   This is logically an example analogous to G  del s theorem       any complete system of axioms is also inconsistent     Ponder the contradictory weirdness of that quote  It is also an example that demonstrates how the expression of semantics does not have a provable bound  thus we can t prove2 that a program  and analogously its semantics  halt a k a  the Halting theorem       The incompleteness theorems derive from the fundamental nature of our universe  which as stated in the Second Law of Thermodynamics is    the entropy  a k a  the   of independent possibilities  is trending to maximum forever     The coding and design of a program is never finished    it s alive     because it attempts to address a real world need  and the semantics of the real world are always changing and trending to more possibilities  Humans never stop discovering new things  including errors in programs  -        To precisely and technically capture this aforementioned desired notion within this weird universe that has no edge  ponder that  there is no    outside    of our universe   requires a terse but deceptively-not-simple definition which will sound incorrect until it is explained deeply       Definition                      The declarative property is where there can exist only one possible set of statements that can express each specific modular semantic           The imperative property3 is the dual  where semantics are inconsistent under composition and or can be expressed with variations of sets of statements                    This definition of declarative is distinctively local in semantic scope  meaning that it requires that a modular semantic maintain its consistent meaning regardless where and how it s instantiated and employed in global scope  Thus each declarative modular semantic should be intrinsically orthogonal to all possible others    and not an impossible  due to incompleteness theorems  global algorithm or model for witnessing consistency  which is also the point of    More Is Not Always Better    by Robert Harper  Professor of Computer Science at Carnegie Mellon University  one of the designers of Standard ML       Examples of these modular declarative semantics include category theory functors e g  the Applicative  nominal typing  namespaces  named fields  and w r t  to operational level of semantics then pure functional programming       Thus well designed declarative languages can more clearly express meaning  albeit with some loss of generality in what can be expressed  yet a gain in what can be expressed with intrinsic consistency       An example of the aforementioned definition is the set of formulas in the cells of a spreadsheet program    which are not expected to give the same meaning when moved to different column and row cells  i e  cell identifiers changed  The cell identifiers are part of and not superfluous to the intended meaning  So each spreadsheet result is unique w r t  to the cell identifiers in a set of formulas  The consistent modular semantic in this case is use of cell identifiers as the input and output of pure functions for cells formulas  see below        Hyper Text Markup Language a k a  HTML    the language for static web pages    is an example of a highly  but not perfectly3  declarative language that  at least before HTML 5  had no capability to express dynamic behavior  HTML is perhaps the easiest language to learn  For dynamic behavior  an imperative scripting language such as JavaScript was usually combined with HTML  HTML without JavaScript fits the declarative definition because each nominal type  i e  the tags  maintains its consistent meaning under composition within the rules of the syntax       A competing definition for declarative is the commutative and idempotent properties of the semantic statements  i e  that statements can be reordered and duplicated without changing the meaning  For example  statements assigning values to named fields can be reordered and duplicated without changed the meaning of the program  if those names are modular w r t  to any implied order  Names sometimes imply an order  e g  cell identifiers include their column and row position    moving a total on spreadsheet changes its meaning  Otherwise  these properties implicitly require global consistency of semantics  It is generally impossible to design the semantics of statements so they remain consistent if randomly ordered or duplicated  because order and duplication are intrinsic to semantics  For example  the statements    Foo exists     or construction  and    Foo does not exist     and destruction   If one considers random inconsistency endemical of the intended semantics  then one accepts this definition as general enough for the declarative property  In essence this definition is vacuous as a generalized definition because it attempts to make consistency orthogonal to semantics  i e  to defy the fact that the universe of semantics is dynamically unbounded and can t be captured in a global coherence paradigm       Requiring the commutative and idempotent properties for the  structural evaluation order of the  lower-level operational semantics converts operational semantics to a declarative localized modular semantic  e g  pure functional programming  including recursion instead of imperative loops   Then the operational order of the implementation details do not impact  i e  spread globally into  the consistency of the higher-level semantics  For example  the order of evaluation of  and theoretically also the duplication of  the spreadsheet formulas doesn t matter because the outputs are not copied to the inputs until after all outputs have been computed  i e  analogous to pure functions       C  Java  C    C   PHP  and JavaScript aren t particularly declarative    Copute s syntax and Python s syntax are more declaratively coupled to   intended results  i e  consistent syntactical semantics that eliminate the extraneous so one can readily   comprehend code after they ve forgotten it  Copute and Haskell enforce   determinism of the operational semantics and encourage    don t repeat   yourself     DRY   because they only allow the pure functional paradigm             2 Even where we can prove the semantics of a program  e g  with the language Coq  this is limited to the semantics that are expressed in the typing  and typing can never capture all of the semantics of a program    not even for languages that are not Turing complete  e g  with HTML CSS it is possible to express inconsistent combinations which thus have undefined semantics       3 Many explanations incorrectly claim that only imperative programming has syntactically ordered statements  I clarified this confusion between imperative and functional programming  For example  the order of HTML statements does not reduce the consistency of their meaning      Edit  I posted the following comment to Robert Harper s blog           in functional programming     the range of variation of a variable is a type         Depending on how one distinguishes functional from imperative   programming  your    assignable    in an imperative program also may have   a type placing a bound on its variability       The only non-muddled definition I currently appreciate for functional   programming is a  functions as first-class objects and types  b    preference for recursion over loops  and or c  pure functions    i e    those functions which do not impact the desired semantics of the   program when memoized  thus perfectly pure functional   programming doesn t exist in a general purpose denotational semantics   due to impacts of operational semantics  e g  memory   allocation        The idempotent property of a pure function means the function call on   its variables can be substituted by its value  which is not generally   the case for the arguments of an imperative procedure  Pure functions   seem to be declarative w r t  to the uncomposed state transitions   between the input and result types       But the composition of pure functions does not maintain any such   consistency  because it is possible to model a side-effect  global   state  imperative process in a pure functional programming language    e g  Haskell s IOMonad and moreover it is entirely impossible to   prevent doing such in any Turing complete pure functional programming   language       As I wrote in 2012 which seems to the similar consensus of   comments in your recent blog  that declarative programming is an   attempt to capture the notion that the intended semantics are never   opaque  Examples of opaque semantics are dependence on order    dependence on erasure of higher-level semantics at the operational   semantics layer  e g  casts are not conversions and reified generics   limit higher-level semantics   and dependence on variable values   which can not be checked  proved correct  by the programming language       Thus I have concluded that only non-Turing complete languages can be   declarative       Thus one unambiguous and distinct attribute of a declarative language   could be that its output can be proven to obey some enumerable set of   generative rules  For example  for any specific HTML program  ignoring   differences in the ways interpreters diverge  that is not scripted    i e  is not Turing complete  then its output variability can be   enumerable  Or more succinctly an HTML program is a pure function of   its variability  Ditto a spreadsheet program is a pure function of its   input variables       So it seems to me that declarative languages are the antithesis of   unbounded recursion  i e  per G  del s second incompleteness   theorem self-referential theorems can t be proven       Lesie Lamport wrote a fairytale about how Euclid might have   worked around G  del s incompleteness theorems applied to math proofs   in the programming language context by to congruence between types and   logic  Curry-Howard correspondence  etc

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imagine an excel page  With columns populated with formulas to calculate you tax return   All the logic is done declared in the cells   the order of the calculation is by determine by formula itself rather than procedurally    That is sort of what declarative programming is all about  You declare the problem space and the solution rather than the flow of the program   Prolog is the only declarative language I ve use  It requires a different kind of thinking but it s good to learn if just to expose you to something other than the typical procedural programming language

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I d explain it as DP is a way to express   A goal expression  the conditions for - what we are searching for  Is there one  maybe or many  Some known facts Rules that extend the know facts      and where there is a deduct engine usually working with a unification algorithm to find the goals

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Declarative programming is the picture  where imperative programming is instructions for painting that picture   You re writing in a declarative style if you re  Telling it what it is   rather than describing the steps the computer should take to get to where you want it   When you use XML to mark-up data  you re using declarative programming because you re saying  This is a person  that is a birthday  and over there is a street address    Some examples of where declarative and imperative programming get combined for greater effect    Windows Presentation Foundation uses declarative XML syntax to describe what a user interface looks like  and what the relationships  bindings  are between controls and underlying data structures  Structured configuration files use declarative syntax  as simple as  key value  pairs  to identify what a string or value of data means  HTML marks up text with tags that describe what role each piece of text has in relation to the whole document

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Declarative programming is when you write your code in such a way that it describes what you want to do  and not how you want to do it  It is left up to the compiler to figure out the how      Examples of declarative programming languages are SQL and Prolog

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Declarative Programming is programming with declarations  i e  declarative sentences  Declarative sentences have a number of properties that distinguish them from imperative sentences  In particular  declarations are    commutative  can be reordered  associative  can be regrouped  idempotent  can repeat without change in meaning  monotonic  declarations don t subtract information    A relevant point is that these are all structural properties and are orthogonal to subject matter  Declarative is not about  What vs  How   We can declare  represent and constrain  a  how  just as easily as we declare a  what   Declarative is about structure  not content  Declarative programming has a significant impact on how we abstract and refactor our code  and how we modularize it into subprograms  but not so much on the domain model   Often  we can convert from imperative to declarative by adding context  E g  from  Turn left       wait for it      Turn Right    to  Bob will turn left at intersection of Foo and Bar at 11 01  Bob will turn right at the intersection of Bar and Baz at 11 06   Note that in the latter case the sentences are idempotent and commutative  whereas in the former case rearranging or repeating the sentences would severely change the meaning of the program   Regarding monotonic  declarations can add constraints which subtract possibilities  But constraints still add information  more precisely  constraints are information   If we need time-varying declarations  it is typical to model this with explicit temporal semantics - e g  from  the ball is flat  to  the ball is flat at time T   If we have two contradictory declarations  we have an inconsistent declarative system  though this might be resolved by introducing soft constraints  priorities  probabilities  etc   or leveraging a paraconsistent logic

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It s a method of programming based around describing what something should do or be instead of describing how it should work   In other words  you don t write algorithms made of expressions  you just layout how you want things to be   Two good examples are HTML and WPF   This Wikipedia article is a good overview  http   en wikipedia org wiki Declarative programming

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I am sorry  but I must disagree with many of the other answers  I would like to stop this muddled misunderstanding of the definition of declarative programming   Definition  Referential transparency  RT  of the sub-expressions is the only required attribute of a declarative programming expression  because it is the only attribute which is not shared with imperative programming   Other cited attributes of declarative programming  derive from this RT  Please click the hyperlink above for the detailed explanation   Spreadsheet example  Two answers mentioned spreadsheet programming  In the cases where the spreadsheet programming  a k a  formulas  does not access mutable global state  then it is declarative programming  This is because the mutable cell values are the monolithic input and output of the main    the entire program   The new values are not written to the cells after each formula is executed  thus they are not mutable for the life of the declarative program  execution of all the formulas in the spreadsheet   Thus relative to each other  the formulas view these mutable cells as immutable  An RT function is allowed to access immutable global state  and also mutable local state    Thus the ability to mutate the values in the cells when the program terminates  as an output from main     does not make them mutable stored values in the context of the rules  The key distinction is the cell values are not updated after each spreadsheet formula is performed  thus the order of performing the formulas does not matter  The cell values are updated after all the declarative formulas have been performed

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Loosely   Declarative programming tends towards -   Sets of declarations  or declarative statements  each of which has meaning  often in the problem domain  and may be understood independently and in isolation    Imperative programming tends towards -    Sequences of commands  each of which perform some action  but which may or may not have meaning in the problem domain    As a result  an imperative style helps the reader to understand the mechanics of what the system is actually doing  but may give little insight into the problem that it is intended to solve  On the other hand  a declarative style helps the reader to understand the problem domain and the approach that the system takes towards the solution of the problem  but is less informative on the matter of mechanics   Real programs  even ones written in languages that favor the ends of the spectrum  such as ProLog or C  tend to have both styles present to various degrees at various points  to satisfy the varying complexities and communication needs of the piece  One style is not superior to the other  they just serve different purposes  and  as with many things in life  moderation is key

[programming-languages] What is declarative programming?

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