45 |
Sierpinski Triangle
46 |
47 | import Diagrams.Backend.Cairo (Cairo)
48 | import Diagrams.Prelude
49 | import Yesod.Media.Simple
50 |
51 | main :: IO ()
52 | main = serve (sierpinski 4)
53 |
54 | sierpinski :: Int -> Diagram Cairo
55 | sierpinski 1 = triangle 1
56 | sierpinski n = s
57 | ===
58 | (s ||| s) # centerX
59 | where s = sierpinski (n-1)
60 |
61 |
Console Calculator
62 |
How would we write a calculator program in Haskell? A simple
63 | calculator should be able to perform the basic arithmetic operations
64 | on two numbers.
65 |
data Operator = Add | Subtract | Multiply | Divide
66 | deriving Read
67 |
68 | eval l o r = case o of
69 | Add -> l + r
70 | Subtract -> l - r
71 | Multiply -> l * r
72 | Divide -> l / r
73 |
74 | prompt txt = do
75 | putStrLn txt
76 | readLn
77 |
78 | main = do
79 | l <- prompt "Left operand?"
80 | o <- prompt "Which operator?"
81 | r <- prompt "Right operand?"
82 | putStrLn $ "The result is " ++ show (eval l o r)
83 |
84 |
Web Calculator
85 |
Let's make our calculator program into a website!
86 |
{-# LANGUAGE TypeFamilies, QuasiQuotes, MultiParamTypeClasses,
87 | TemplateHaskell, OverloadedStrings #-}
88 | import Yesod hiding (Add, Subtract, Multiply, Divide)
89 | import Yesod.Form
90 | import Control.Applicative
91 |
92 | data Calculator = Calculator
93 |
94 | instance Yesod Calculator
95 |
96 | instance RenderMessage Calculator FormMessage where
97 | renderMessage _ _ = defaultFormMessage
98 |
99 | mkYesod "Calculator" [parseRoutes|
100 | / HomeR GET
101 | |]
102 |
103 | main = warpEnv Calculator
104 |
105 | data Calculation = Calculation Double Operator Double
106 |
107 | form = renderDivs $ Calculation
108 | <$> areq doubleField "Left operand" Nothing
109 | <*> areq (selectField optionsEnum) "Operator" Nothing
110 | <*> areq doubleField "Right operand" Nothing
111 |
112 | getHomeR = do
113 | ((result, widget), enctype) <- runFormGet form
114 |
115 | let resultText = "Result: " ++ case result of
116 | FormSuccess (Calculation l o r) -> show (eval l o r)
117 | _ -> ""
118 |
119 | defaultLayout $ do
120 | [whamlet|
121 | <h1> Calculator </h1>
122 | <form enctype=#{enctype}>
123 | ^{widget}
124 | <input #button type="submit" value="Calculate!">
125 | <h2> #{toHtml resultText}
126 | |]
127 | toWidget [cassius|
128 | body { width: 15em; margin: 0em auto; }
129 | #button { width: 10em; }
130 | input, select { width: 4em; margin: 0.5em; }
131 | .required { text-align: right; }
132 | |]
133 |
134 | data Operator = Add | Subtract | Multiply | Divide
135 | deriving (Eq, Enum, Bounded, Show)
136 |
137 | eval l o r = case o of
138 | Add -> l + r
139 | Subtract -> l - r
140 | Multiply -> l * r
141 | Divide -> l / r
142 |
143 |