datamatrix net example CREATING YOUR FIRST F# PROGRAM in VB.NET

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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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Pipelining with |>
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The |> forward pipe operator is perhaps the most important operator in F# programming. Its definition is deceptively simple:
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let (|>) x f = f x
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Here is how to use the operator to compute the cubes of three numbers:
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[1;2;3] |> List.map (fun x -> x * x * x)
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This produces [1;8;27], just as if you had written this:
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List.map (fun x -> x * x * x) [1;2;3]
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In a sense, |> is function application in reverse. However, using |> has distinct advantages: Clarity: When used in conjunction with operators such as List.map, the |> operator allows you to perform the data transformations and iterations in a forward-chaining, pipelined style. Type inference: Using the |> operator lets type information flow from input objects to the functions manipulating those objects. F# uses information collected from type inference to resolve some language constructs such as property accesses and method overloading. This relies on information being propagated left to right through the text of a program. In particular, typing information to the right of a position isn t taken into account when resolving property access and overloads.
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For completeness, here is the type of the operator:
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val (|>) : 'T -> ('T -> 'U) -> 'U
Composing Functions with >>
You saw earlier how to use the |> forward pipe operator to pipe values through a number of functions. This was a small example of the process of computing with functions, an essential and powerful programming technique in F#. This section covers ways to compute new function values from existing ones using compositional techniques. First, let s look at function composition. For example, consider the following code: let google = http "http://www.google.com" google |> getWords |> List.filter (fun s -> s = "href") |> List.length You can rewrite this code using function composition as follows: let countLinks = getWords >> List.filter (fun s -> s = "href") >> List.length google |> countLinks You define countLinks as the composition of three function values using the >> forward composition operator. This operator is defined in the F# library as follows:
CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
let (>>) f g x = g(f(x)) You can see from the definition that f >> g gives a function value that first applies f to the x and then applies g. Here is the type of >>: val (>>) : ('T -> 'U) -> ('U -> 'c) -> ('T -> 'c) Note that >> is typically applied to only two arguments: those on either side of the binary operator, here named f and g. The final argument x is typically supplied at a later point. F# is good at optimizing basic constructions of pipelines and composition sequences from functions for example, the function countLinks shown earlier becomes a single function calling the three functions in the pipeline in sequence. This means sequences of compositions can be used with relatively low overhead.
Building Functions with Partial Application
Composing functions is just one way to compute interesting new functions. Another useful way is to use partial application. Here s an example: let let let let let shift (dx,dy) (px,py) = (px + dx, py + dy) shiftRight = shift (1,0) shiftUp = shift (0,1) shiftLeft = shift (-1,0) shiftDown = shift (0,-1)
The last four functions are defined by calling shift with only one argument, in each case leaving a residue function that expects an additional argument. F# Interactive reports the types as follows: val val val val val shift : int * int -> int * int -> int * int shiftRight : int * int -> int * int shiftLeft : int * int -> int * int shiftUp : int * int -> int * int shiftDown : int * int -> int * int Here is an example of how to use shiftRight and how to apply shift to new arguments (2,2): > shiftRight (10,10);; val it : int * int = (11,10) > List.map (shift (2,2)) [ (0,0); (1,0); (1,1); (0,1) ];; val it : int * int list = [ (2,2); (3,2); (3,3); (2,3) ] In the second example, the function shift takes two pairs as arguments. You bind the first parameter to (2, 2). The result of this partial application is a function that takes one remaining tuple parameter and returns the value shifted by two units in each direction. This resulting function can now be used in conjunction with List.map.
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