Functional Declarative and Imperative Programming

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What do the terms functional  declarative  and imperative programming mean

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In a nutshell   An imperative language specfies a series of instructions that the computer executes in sequence  do this  then do that    A declarative language declares a set of rules about what outputs should result from which inputs  eg  if you have A  then the result is B    An engine will apply these rules to inputs  and give an output   A functional language declares a set of mathematical logical functions which define how input is translated to output   eg  f y    y   y   it is a type of declarative language

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Imperative programming  telling the    machine    how to do something  and as a result what you want to happen will happen   Declarative programming  telling the    machine    what you would like to happen  and letting the computer figure out how to do it   Example of imperative  function makeWidget options        const element   document createElement  div        element style backgroundColor   options bgColor      element style width   options width      element style height   options height      element textContent   options txt       return element      Example of declarative  function makeWidget type  txt        return new Element type  txt       Note  The difference is not one of brevity or complexity or abstraction  As stated  the difference is how vs what

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At the time of writing this  the top voted answers on this page are imprecise and muddled on the declarative vs  imperative definition  including the answer that quotes Wikipedia  Some answers are conflating the terms in different ways  Refer also to my explanation of why spreadsheet programming is declarative  regardless that the formulas mutate the cells  Also  several answers claim that functional programming must be a subset of declarative  On that point it depends if we differentiate  quot function quot  from  quot procedure quot   Lets handle imperative vs  declarative first  Definition of declarative expression The only attribute that can possibly differentiate a declarative expression from an imperative expression is the referential transparency  RT  of its sub-expressions  All other attributes are either shared between both types of expressions  or derived from the RT  A 100  declarative language  i e  one in which every possible expression is RT  does not  among other RT requirements  allow the mutation of stored values  e g  HTML and most of Haskell  Definition of RT expression RT is often referred to as having  quot no side-effects quot   The term effects does not have a precise definition  so some people don t agree that  quot no side-effects quot  is the same as RT  RT has a precise definition   An expression e is referentially transparent if for all programs p every occurrence of e in p can be replaced with the result of evaluating e  without affecting the observable result of p   Since every sub-expression is conceptually a function call  RT requires that the implementation of a function  i e  the expression s  inside the called function  may not access the mutable state that is external to the function  accessing the mutable local state is allowed   Put simply  the function  implementation  should be pure  Definition of pure function A pure function is often said to have  quot no side-effects quot   The term effects does not have a precise definition  so some people don t agree  Pure functions have the following attributes   the only observable output is the return value  the only output dependency is the arguments  arguments are fully determined before any output is generated   Remember that RT applies to expressions  which includes function calls  and purity applies to   implementations of  functions  An obscure example of impure functions that make RT expressions is concurrency  but this is because the purity is broken at the interrupt abstraction layer  You don t really need to know this  To make RT expressions  you call pure functions  Derivative attributes of RT Any other attribute cited for declarative programming  e g  the citation from 1999 used by Wikipedia  either derives from RT  or is shared with imperative programming  Thus proving that my precise definition is correct  Note  immutability of external values is a subset of the requirements for RT   Declarative languages don t have looping control structures  e g  for and while  because due to immutability  the loop condition would never change   Declarative languages don t express control-flow other than nested function order  a k a logical dependencies   because  due to immutability  other choices of evaluation order do not change the result  see below    Declarative languages express logical  quot steps quot   i e  the nested RT function call order   but whether each function call is a higher level semantic  i e   quot what to do quot   is not a requirement of declarative programming  The distinction from imperative is that due to immutability  i e  more generally RT   these  quot steps quot  cannot depend on mutable state  rather only the relational order of the expressed logic  i e  the order of nesting of the function calls  a k a  sub-expressions     For example  the HTML paragraph  lt p gt  cannot be displayed until the sub-expressions  i e  tags  in the paragraph have been evaluated  There is no mutable state  only an order dependency due to the logical relationship of tag hierarchy  nesting of sub-expressions  which are analogously nested function calls    Thus there is the derivative attribute of immutability  more generally RT   that declarative expressions  express only the logical relationships of the constituent parts  i e  of the sub-expression function arguments  and not mutable state relationships   Evaluation order The choice of evaluation order of sub-expressions can only give a varying result when any of the function calls are not RT  i e  the function is not pure   e g  some mutable state external to a function is accessed within the function  For example  given some nested expressions  e g  f  g a  b   h c  d     eager and lazy evaluation of the function arguments will give the same results if the functions f  g  and h are pure  Whereas  if the functions f  g  and h are not pure  then the choice of evaluation order can give a different result  Note  nested expressions are conceptually nested functions  since expression operators are just function calls masquerading as unary prefix  unary postfix  or binary infix notation  Tangentially  if all identifiers  e g  a  b  c  d  are immutable everywhere  state external to the program cannot be accessed  i e  I O   and there is no abstraction layer breakage  then functions are always pure  By the way  Haskell has a different syntax  f  g a b   h c d   Evaluation order details A function is a state transition  not a mutable stored value  from the input to the output  For RT compositions of calls to pure functions  the order-of-execution of these state transitions is independent  The state transition of each function call is independent of the others  due to lack of side-effects and the principle that an RT function may be replaced by its cached value  To correct a popular misconception  pure monadic composition is always declarative and RT  in spite of the fact that Haskell s IO monad is arguably impure and thus imperative w r t  the World state external to the program  but in the sense of the caveat below  the side-effects are isolated   Eager evaluation means the functions arguments are evaluated before the function is called  and lazy evaluation means the arguments are not evaluated until  and if  they are accessed within the function  Definition  function parameters are declared at the function definition site  and function arguments are supplied at the function call site  Know the difference between parameter and argument  Conceptually  all expressions are  a composition of  function calls  e g  constants are functions without inputs  unary operators are functions with one input  binary infix operators are functions with two inputs  constructors are functions  and even control statements  e g  if  for  while  can be modeled with functions  The order that these argument functions  do not confuse with nested function call order  are evaluated is not declared by the syntax  e g  f  g     could eagerly evaluate g then f on g s result or it could evaluate f and only lazily evaluate g when its result is needed within f  Caveat  no Turing complete language  i e  that allows unbounded recursion  is perfectly declarative  e g  lazy evaluation introduces memory and time indeterminism  But these side-effects due to the choice of evaluation order are limited to memory consumption  execution time  latency  non-termination  and external hysteresis thus external synchronization  Functional programming Because declarative programming cannot have loops  then the only way to iterate is functional recursion  It is in this sense that functional programming is related to declarative programming  But functional programming is not limited to declarative programming  Functional composition can be contrasted with subtyping  especially with respect to the Expression Problem  where extension can be achieved by either adding subtypes or functional decomposition  Extension can be a mix of both methodologies  Functional programming usually makes the function a first-class object  meaning the function type can appear in the grammar anywhere any other type may  The upshot is that functions can input and operate on functions  thus providing for separation-of-concerns by emphasizing function composition  i e  separating the dependencies among the subcomputations of a deterministic computation  For example  instead of writing a separate function  and employing recursion instead of loops if the function must also be declarative  for each of an infinite number of possible specialized actions that could be applied to each element of a collection  functional programming employs reusable iteration functions  e g  map  fold  filter  These iteration functions input a first-class specialized action function  These iteration functions iterate the collection and call the input specialized action function for each element  These action functions are more concise because they no longer need to contain the looping statements to iterate the collection  However  note that if a function is not pure  then it is really a procedure  We can perhaps argue that functional programming that uses impure functions  is really procedural programming  Thus if we agree that declarative expressions are RT  then we can say that procedural programming is not declarative programming  and thus we might argue that functional programming is always RT and must be a subset of declarative programming  Parallelism This functional composition with first-class functions can express the depth in the parallelism by separating out the independent function   Brent   s Principle  computation with work w and depth d can be implemented in a p-processor PRAM in time O max w p  d     Both concurrency and parallelism also require declarative programming  i e  immutability and RT   So where did this dangerous assumption that Parallelism    Concurrency come from   It   s a natural consequence of languages with side-effects  when your language has side-effects everywhere  then any time you try to do more than one thing at a time you essentially have non-determinism caused by the interleaving of the effects from each operation   So in side-effecty languages  the only way to get parallelism is concurrency  it   s therefore not surprising that we often see the two conflated   FP evaluation order Note the evaluation order also impacts the termination and performance side-effects of functional composition  Eager  CBV  and lazy  CBN  are categorical duels 10   because they have reversed evaluation order  i e  whether the outer or inner functions respectively are evaluated first  Imagine an upside-down tree  then eager evaluates from function tree branch tips up the branch hierarchy to the top-level function trunk  whereas  lazy evaluates from the trunk down to the branch tips  Eager doesn t have conjunctive products   quot and quot   a k a categorical  quot products quot   and lazy doesn t have disjunctive coproducts   quot or quot   a k a categorical  quot sums quot   11   Performance  Eager  As with non-termination  eager is too eager with conjunctive functional composition  i e  compositional control structure does unnecessary work that isn t done with lazy  For example  eager eagerly and unnecessarily maps the entire list to booleans  when it is composed with a fold that terminates on the first true element  This unnecessary work is the cause of the claimed  quot up to quot  an extra log n factor in the sequential time complexity of eager versus lazy  both with pure functions  A solution is to use functors  e g  lists  with lazy constructors  i e  eager with optional lazy products   because with eager the eagerness incorrectness originates from the inner function  This is because products are constructive types  i e  inductive types with an initial algebra on an initial fixpoint 11   Lazy  As with non-termination  lazy is too lazy with disjunctive functional composition  i e  coinductive finality can occur later than necessary  resulting in both unnecessary work and non-determinism of the lateness that isn t the case with eager 10  11   Examples of finality are state  timing  non-termination  and runtime exceptions  These are imperative side-effects  but even in a pure declarative language  e g  Haskell   there is state in the imperative IO monad  note  not all monads are imperative   implicit in space allocation  and timing is state relative to the imperative real world  Using lazy even with optional eager coproducts leaks  quot laziness quot  into inner coproducts  because with lazy the laziness incorrectness originates from the outer function  see the example in the Non-termination section  where    is an outer binary operator function   This is because coproducts are bounded by finality  i e  coinductive types with a final algebra on an final object 11   Lazy causes indeterminism in the design and debugging of functions for latency and space  the debugging of which is probably beyond the capabilities of the majority of programmers  because of the dissonance between the declared function hierarchy and the runtime order-of-evaluation  Lazy pure functions evaluated with eager  could potentially introduce previously unseen non-termination at runtime  Conversely  eager pure functions evaluated with lazy  could potentially introduce previously unseen space and latency indeterminism at runtime  Non-termination At compile-time  due to the Halting problem and mutual recursion in a Turing complete language  functions can t generally be guaranteed to terminate   Eager  With eager but not lazy  for the conjunction of Head  quot and quot  Tail  if either Head or Tail doesn t terminate  then respectively either List  Head    Tail     tail    Tail   or List  Head    Tail     head    Head   is not true because the left-side doesn t  and right-side does  terminate  Whereas  with lazy both sides terminate  Thus eager is too eager with conjunctive products  and non-terminates  including runtime exceptions  in those cases where it isn t necessary   Lazy  With lazy but not eager  for the disjunction of 1  quot or quot  2  if f doesn t terminate  then List  f   1   2  3   tail     f   List  1  3     List  2  3    tail is not true because the left-side terminates  and right-side doesn t  Whereas  with eager neither side terminates so the equality test is never reached  Thus lazy is too lazy with disjunctive coproducts  and in those cases fails to terminate  including runtime exceptions  after doing more work than eager would have   10  Declarative Continuations and Categorical Duality  Filinski  sections 2 5 4 A comparison of CBV and CBN  and 3 6 1 CBV and CBN in the SCL   11  Declarative Continuations and Categorical Duality  Filinski  sections 2 2 1 Products and coproducts  2 2 2 Terminal and initial objects  2 5 2 CBV with lazy products  and 2 5 3 CBN with eager coproducts

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In a nutshell  the more a programming style emphasizes What  to do  abstracting away the details of How  to do it  the more that style is considered to be declarative   The opposite is true for imperative   Functional programming is associated with the declarative style

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imperative - expressions describe sequence of actions to perform  associative   declarative - expressions are declarations that contribute to behavior of program  associative  commutative  idempotent  monotonic   functional - expressions have value as only effect  semantics support equational reasoning

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Imperative  how to achieve our goal     Take the next customer from a list     If the customer lives in Spain  show their details     If there are more customers in the list  go to the beginning   Declarative  what we want to achieve     Show customer details of every customer living in Spain

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I think that your taxonomy is incorrect  There are two opposite types imperative and declarative  Functional is just a subtype of declarative  BTW  wikipedia states the same fact

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imperative and declarative describe two opposing styles of programming   imperative is the traditional  step by step recipe  approach while declarative is more  this is what i want  now you work out how to do it    these two approaches occur throughout programming - even with the same language and the same program   generally the declarative approach is considered preferable  because it frees the programmer from having to specify so many details  while also having less chance for bugs  if you describe the result you want  and some well-tested automatic process can work backwards from that to define the steps then you might hope that things are more reliable than having to specify each step by hand    on the other hand  an imperative approach gives you more low level control - it s the  micromanager approach  to programming   and that can allow the programmer to exploit knowledge about the problem to give a more efficient answer   so it s not unusual for some parts of a program to be written in a more declarative style  but for the speed-critical parts to be more imperative   as you might imagine  the language you use to write a program affects how declarative you can be - a language that has built-in  smarts  for working out what to do given a description of the result is going to allow a much more declarative approach than one where the programmer needs to first add that kind of intelligence with imperative code before being able to build a more declarative layer on top   so  for example  a language like prolog is considered very declarative because it has  built-in  a process that searches for answers   so far  you ll notice that i haven t mentioned functional programming   that s because it s a term whose meaning isn t immediately related to the other two   at its most simple  functional programming means that you use functions   in particular  that you use a language that supports functions as  first class values  - that means that not only can you write functions  but you can write functions that write functions  that write functions that      and pass functions to functions   in short - that functions are as flexible and common as things like strings and numbers   it might seem odd  then  that functional  imperative and declarative are often mentioned together   the reason for this is a consequence of taking the idea of functional programming  to the extreme    a function  in it s purest sense  is something from maths - a kind of  black box  that takes some input and always gives the same output   and that kind of behaviour doesn t require storing changing variables   so if you design a programming language whose aim is to implement a very pure  mathematically influenced idea of functional programming  you end up rejecting  largely  the idea of values that can change  in a certain  limited  technical sense    and if you do that - if you limit how variables can change - then almost by accident you end up forcing the programmer to write programs that are more declarative  because a large part of imperative programming is describing how variables change  and you can no longer do that   so it turns out that functional programming - particularly  programming in a functional language - tends to give more declarative code   to summarise  then    imperative and declarative are two opposing styles of programming  the same names are used for programming languages that encourage those styles  functional programming is a style of programming where functions become very important and  as a consequence  changing values become less important   the limited ability to specify changes in values forces a more declarative style    so  functional programming  is often described as  declarative

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Nowadays  new focus  we need the old  classifications   The  Imperative Declarative Functional aspects was good in the past to classify generic languages  but in nowadays all  big language   as Java  Python  Javascript  etc   have some option  typically frameworks  to  express with  other focus  than its main one  usual imperative   and to express parallel processes  declarative functions  lambdas  etc   So a good variant of this question is  What aspect is good  to classify frameworks today       An important aspect is something that we can labeling  programming style       Focus on the fusion of data with algorithm  A good example to explain  As you can read about jQuery at Wikipedia      The set of jQuery core features     DOM element selections  traversal and manipulation      enabled by its selector engine        created a new  programming style   fusing algorithms and DOM-data-structures   So jQuery is the best  popular  example of focusing on a  new programming style   that is not only object orientation  is  Fusing algorithms and data-structures   jQuery is somewhat reactive as spreadsheets  but not  cell-oriented   is  DOM-node oriented     Comparing the main styles in this context    No fusion  in all  big languages   in any Functional Declarative Imperative expression  the usual is  no fusion  of data and algorithm  except by some object-orientation  that is a fusion in strict algebric structure point of view  Some fusion  all classic strategies of fusion  in nowadays have some framework using it as paradigm     dataflow    Event-driven programming  or old domain specific languages as awk and XSLT     Like programming with modern spreadsheets  they are also examples of reactive programming style  Big fusion  is  the jQuery style     jQuery is a domain specific language focusing on  fusing algorithms and DOM-data-structures    PS  other  query languages   as XQuery   SQL  with PL as imperative expression option  are also data-algorith-fusion examples  but they are islands  with no fusion with other system modules    Spring  when using  find  -variants and Specification clauses  is another good fusion example

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Declarative programming is programming by expressing some timeless logic between the input and the output  for instance  in pseudocode  the following example would be declarative   def factorial n     if n  lt  2      return 1   else      return factorial n-1   output   factorial argvec 0     We just define a relationship called the  factorial  here  and defined the relationship between the output and the input as the that relationship  As should be evident here  about any structured language allows declarative programming to some extend  A central idea of declarative programming is immutable data  if you assign to a variable  you only do so once  and then never again  Other  stricter definitions entail that there may be no side-effects at all  these languages are some times called  purely declarative    The same result in an imperative style would be   a   1 b   argvec 0  while b  lt  2     a   b--  output   a   In this example  we expressed no timeless static logical relationship between the input and the output  we changed memory addresses manually until one of them held the desired result  It should be evident that all languages allow declarative semantics to some extend  but not all allow imperative  some  purely  declarative languages permit side effects and mutation altogether   Declarative languages are often said to specify  what must be done   as opposed to  how to do it   I think that is a misnomer  declarative programs still specify how one must get from input to output  but in another way  the relationship you specify must be effectively computable  important term  look it up if you don t know it   Another approach is nondeterministic programming  that really just specifies what conditions a result much meet  before your implementation just goes to exhaust all paths on trial and error until it succeeds   Purely declarative languages include Haskell and Pure Prolog  A sliding scale from one and to the other would be  Pure Prolog  Haskell  OCaml  Scheme Lisp  Python  Javascript  C--  Perl  PHP  C    Pascall  C  Fortran  Assembly

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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   All exotic types of programming fit into the following taxonomy of declarative versus imperative  since the following definition claims they are duals      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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Imperative Programming means any style of programming where your program is structured out of instructions describing how the operations performed by a computer will happen   Declarative Programming means any style of programming where your program is a description either of the problem or the solution - but doesn t explicitly state how the work will be done   Functional Programming is programming by evaluating functions and functions of functions    As  strictly defined  functional programming means programming by defining side-effect free mathematical functions so it is a form of declarative programming but it isn t the only kind of declarative programming   Logic Programming  for example in Prolog  is another form of declarative programming  It involves computing by deciding whether a logical statement is true  or whether it can be satisfied   The program is typically a series of facts and rules - i e  a description rather than a series of instructions   Term Rewriting  for example CASL  is another form of declarative programming  It involves symbolic transformation of algebraic terms  It s completely distinct from logic programming and functional programming

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There s not really any non-ambiguous  objective definition for these   Here is how I would define them   Imperative - The focus is on what steps the computer should take rather than what the computer will do  ex  C  C    Java    Declarative - The focus is on what the computer should do rather than how it should do it  ex  SQL    Functional - a subset of declarative languages that has heavy focus on recursion

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Some good answers here regarding the noted  types    I submit some additional  more  exotic  concepts often associated with the functional programming crowd    Domain Specific Language or DSL Programming  creating a new language to deal with the problem at hand  Meta-Programming  when your program writes other programs  Evolutionary Programming  where you build a system that continually improves itself or generates successively better generations of sub-programs

[terminology] Functional, Declarative, and Imperative Programming

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