计算机代考程序代写 compiler Haskell interpreter PROGRAMMING IN HASKELL – cscodehelp代写

PROGRAMMING IN HASKELL
Chapter 2 – First Steps
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Glasgow Haskell Compiler
❚ GHC is the leading implementation of Haskell, and comprises a compiler and interpreter;
❚ The interactive nature of the interpreter makes it well suited for teaching and prototyping;
❚ GHC is freely available from: www.haskell.org/platform
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Starting GHCi
The interpreter can be started from the terminal command prompt $ by simply typing ghci:
$ ghci
GHCi, version X: http://www.haskell.org/ghc/ 😕 for help
Prelude>
The GHCi prompt > means that the interpreter is now ready to evaluate an expression.
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For example, it can be used as a desktop calculator to evaluate simple numeric expresions:
> 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.
fa+b
Means (f a) + b, rather than f (a + b).
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Examples
Mathematics Haskell
f(x) fx f(x,y) f x y f(g(x)) f (g x) f(x,g(y)) f x (g y) f(x)g(y) fx*gy
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❚ 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
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:
double x = x + x
quadruple x = double (double x)
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Leaving the editor open, in another window start up GHCi with the new script:
$ ghci test.hs
Now both the standard library and the file test.hs are loaded, and functions from both can be used:
> quadruple 10
40
> take (double 2) [1,2,3,4,5,6]
[1,2,3,4]
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Leaving GHCi open, return to the editor, add the following two definitions, and resave:
factorial n = product [1..n]
average ns = sum ns `div` length ns
Note:
❚ div is enclosed in back quotes, not forward;
❚ x `f` y is just syntactic sugar for f x y.
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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:
> :reload
Reading file “test.hs”
> factorial 10
3628800
> average [1,2,3,4,5]
3
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Useful GHCi Commands
Command
:load name :reload
:set editor name :edit name
:edit
:type expr
😕
:quit
Meaning
load script name reload current script set editor to name edit script name edit current script show type of expr show all commands quit GHCi
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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
means
implicit grouping
explicit grouping
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Exercises
(1) Try out slides 2-7 and 13-16 using GHCi.
(2) Fix the syntax errors in the program below, and test your solution using GHCi.
N = a ’div’ length xs where
a = 10
xs = [1,2,3,4,5]
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(3) Show how the library function last that selects the last element of a list can be defined using the functions introduced in this lecture.
(4) Can you think of another possible definition?
(5) Similarly, show how the library function init that removes the last element from a list can be defined in two different ways.
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