module test(main) where import System.IO data Shape = Circle Float Float Float | Rectangle Float Float Float Float surface :: Shape -> Float surface (Circle _ _ r) = pi * r ^ 2 surface (Rectangle a b c d) = (a-c) * (b-d) main :: IO() main = do print $ surface $ Circle 2.2 3.0 2.5 print $ surface $ Rectangle 1 2 4 9
编译报错
ghc -o test test.hs test.hs:1:8: parse error on input ‘test’
貌似是因为module名字首字母要大些。
改了之后,在运行。还是有问题,这次是警告。
引用
ghc -o test test.hs
[1 of 1] Compiling Test ( test.hs, test.o )
Warning: output was redirected with -o, but no output will be generated
because there is no Main module.
我比较了网上的hello world例子发现,和我的不同之处在于它的没有定义module。
去掉module那行试一下
引用
[1 of 1] Compiling Main ( test.hs, test.o )
Linking test ...
这次正常了。
运行:
引用
./test
19.634954
21.0
最终版本的代码:
import System.IO data Shape = Circle Float Float Float | Rectangle Float Float Float Float surface :: Shape -> Float surface (Circle _ _ r) = pi * r ^ 2 surface (Rectangle a b c d) = (a-c) * (b-d) main :: IO() main = do print $ surface $ Circle 2.2 3.0 2.5 print $ surface $ Rectangle 1 2 4 9