-ML is a family of StaticallyTyped[1], [strict | StrictEvaluation] [4] [higher-order | HigherOrderFunctions][2] , polymorphic[3], [FunctionalLanguage]s with and a higher-order module system[5]. ML is very good general purpose programming language[6] with a strength in pattern matching[7]. ML can be used interactively for learning, experimentation and testing, or it can be compiled. The two major dialects of ML are [Ocaml] and [SML]. SML is a standardised language[8] with several implementations[9]. Ocaml has a single open source implementation[10], it extends ML with an OOP system[11]. Both major dialects have compilers that produce native code that rivals the speed of C++, and extensive standard[12] and third-party[13] libraries. 

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-[2]  

-!!!Polymorphism  

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-PolymorphicTypes are what prevents ML's strong type checking from being a pain in the bum. You don't have to define the type of everything ''exactly'', you can leave some types, or parts of some types unspecified. This allowed you write a great variety of type-safe generic functions.  

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-[3]  

-!!!Higher-order Functions  

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-Higher-order functions are functions that take other functions as parameters, create functions or return functions.  

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-I want make new trees from old ones by changing the leaves. So I write a function to do this for me. One of its parameters will be a function that takes the value of a leaf and returns the changed value.  

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- # __let rec__ map change tree =  

- __match__ t __with__  

- | Leaf value -> Leaf (change value)  

- | Node (left, right) -> Node (map change left, map change right) ;;  

- ''val map : ('a -> 'b) -> 'a tree -> 'b tree = <fun>''  

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-Once again Ocaml has worked out the types for itself. (__a -> b -> c__ is ML for "function that takes parameters of the types __a__ and __b__ and returns a value of type __c__.) It's also noted that if the __change__ function doesn't return the same type as its given then my __map__ function will return a tree of a different type. Good. That's what I wanted. (If it wasn't I could add some type annotations to say so.)  

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-Here's a function I could use as a __change__ parameter:  

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- # string_of_int ;;  

- ''- : int -> string = <fun>''  

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-You can guess what that does. __map__'s type is __('a -> 'b) -> 'a tree -> 'b tree__, so if I give it __string_of_int__ as its first parameter then __'a__ will be __string__ and __'b__ will be __int__, so __map__ should return a __string tree__...  

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- # __let__ numtree = Node (Node (Leaf 1, Leaf 2), Leaf 3) ;;  

- ''val numtree : int tree = Node (Node (Leaf 1, Leaf 2), Leaf 3)''  

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- # map string_of_int numtree ;;  

- ''- : string tree = Node (Node (Leaf "1", Leaf "2"), Leaf "3")''  

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-And it does!  

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-If I write a few more functions like this I will have a reusable library of binary tree operations. ML makes writing reusable code and easy and ''reliable'' process.  

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-There's a lot more things you can do with higher-order functions, some of them very painful. I've just shown you there's at least ''one'' good thing you can do with then here.  

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-''Higher-order functions are something ML has in common with [Haskell].''  

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- [4]  

-!!!Strict Evaluation  

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- ML uses StrictEvaluation. This means that ML can allow reassignable variables and a conventional I/O system -- and it does. 

-[5]  

-!!! Higher-Order Module System 

-[6]  

-!!! ML is very good general purpose programming language 

-[7]  

-!!!Pattern Matching  

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- Pattern matching is ML's strength. ML has an expression that is halfway between [C]'s__switch__ and declaration statements. In SML it is called __case__ and in Ocaml it is called __match__. Instead matching a value with single constants, like the __switch__ statement, __match__ can match values of any complexity by example, and select parts of the value to assign to variables as it does it. 

-[8]  

-!!! SML standard  

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- The language and standard library of SML are formally defined in the book [Definition of Standard ML| http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=3874]. (This is unusually thorough.) 

-[9]  

-!!! SML Implementations 

-[10]  

-!!! The Ocaml Implementation 

-approach works well once the community user is large enough. Ocaml's user community user has been the necessary size for many years. 

-[11]  

-!!! Ocaml OOP system 

-One of Ocaml's extentions to the original ML language is an [object oriented programming system| ]. Ocaml's OOP has everything you'd expect after using [Java] or [C++]. The syntax is quite different; class declarations are much more compact.  

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- A big improvement is that container classes can be parametized . In C++ and Java container objects only recognise objects inside themselves as being members of the __Object__ class. You have to cast objects back the appropriate class when you remove them from the container. In Ocaml, if you want a container object to specifically contain objects that are of the class __fruitbat__, you can say so. 

-[12]  

-!!! Standard Libraries  

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-The [SML Basis Library | http://cm.bell-labs.com/cm/cs/what/smlnj/doc/basis/pages/sml-std-basis.html] is said to be very well designed . 

-[13]  

-!!! Third-Party Libraries  

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- [The Caml Link Database | http://www.npc.de/ocaml/linkdb/] and [The Hump | http://caml.inria.fr/humps/index.html] are the central repositories for [Ocaml] software.