Functional Programming
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Starting GHC
% ghci
GHCi, version 7.4.1: http://www.haskell.org/ghc/ 😕 for help
Loading package ghc-prim … linking … done.
Loading package integer-gmp … linking … done.
Loading package base … linking … done.
Prelude>
The GHC interpreter can be started from the Unix command prompt % by simply typing ghci:
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The GHCi prompt > means that the interpreter is ready to evaluate an expression.
For example:
> 2+3*4
14
> (2+3)*4
20
> sqrt (3^2 + 4^2)
5.0
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The Standard Prelude
Haskell comes with a large number of standard library functions. In addition to the familiar numeric functions such as + and *, the library also provides many useful functions on lists.
Select the first element of a list:
> head [1,2,3,4,5]
1
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Remove the first element from a list:
> tail [1,2,3,4,5]
[2,3,4,5]
Select the nth element of a list:
> [1,2,3,4,5] !! 2
3
Select the first n elements of a list:
> take 3 [1,2,3,4,5]
[1,2,3]
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Remove the first n elements from a list:
> drop 3 [1,2,3,4,5]
[4,5]
Calculate the length of a list:
> length [1,2,3,4,5]
5
Calculate the sum of a list of numbers:
> sum [1,2,3,4,5]
15
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Calculate the product of a list of numbers:
> product [1,2,3,4,5]
120
Append two lists:
> [1,2,3] ++ [4,5]
[1,2,3,4,5]
Reverse a list:
> reverse [1,2,3,4,5]
[5,4,3,2,1]
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Function Application
In mathematics, function application is denoted using parentheses, and multiplication is often denoted using juxtaposition or space.
f(a,b) + c d
Apply the function f to a and b, and add the result to the product of c and d.
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In Haskell, function application is denoted using space, and multiplication is denoted using *.
f a b + c*d
As previously, but in Haskell syntax.
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Moreover, function application is assumed to have higher priority than all other operators.
f a + b
Means (f a) + b, rather than f (a + b).
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Examples
Mathematics
Haskell
f(x)
f(x,y)
f(g(x))
f(x,g(y))
f(x)g(y)
f x
f x y
f (g x)
f x (g y)
f x * g y
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Haskell Scripts
As well as the functions in the standard library, you can also define your own functions;
New functions are defined within a script, a text file comprising a sequence of definitions;
By convention, Haskell scripts usually have a .hs suffix on their filename. This is not mandatory, but is useful for identification purposes.
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My First Script
double x = x + x
quadruple x = double (double x)
When developing a Haskell script, it is useful to keep two windows open, one running an editor for the script, and the other running GHCi.
Start an editor, type in the following two function definitions, and save the script as test.hs:
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% ghci test.hs
Leaving the editor open, in another window start up GHCi with the new script:
> quadruple 10
40
> take (double 2) [1,2,3,4,5,6]
?
Now both the standard library and the file test.hs are loaded, and functions from both can be used:
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factorial n = product [1..n]
average ns = sum ns `div` length ns
Leaving GHCi open, return to the editor, add the following two definitions, and resave:
div is enclosed in back quotes, not forward;
x `f` y is just syntactic sugar for f x y.
Note:
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> :reload
Reading file “test.hs”
> factorial 10
3628800
> average [1,2,3,4,5]
3
GHCi does not automatically detect that the script has been changed, so a reload command must be executed before the new definitions can be used:
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Naming Requirements
Function and argument names must begin with a lower-case letter. For example:
myFun
fun1
arg_2
x’
By convention, list arguments usually have an s suffix on their name. For example:
xs
ns
nss
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The Layout Rule
In a sequence of definitions, each definition must begin in precisely the same column:
a = 10
b = 20
c = 30
a = 10
b = 20
c = 30
a = 10
b = 20
c = 30
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The layout rule avoids the need for explicit syntax to indicate the grouping of definitions.
a = b + c
where
b = 1
c = 2
d = a * 2
a = b + c
where
{b = 1;
c = 2}
d = a * 2
implicit grouping
explicit grouping
means