Haskell：自定义数据类型中的函数

Question

if I have a custom data type that takes in a string representation of a Boolean (ie "true" or "false" ). How do I make by data type convert that to a Bool with out having to perform an actions on the input before?

for example

λ: MyData "false"
MyData False

Answer 1

You can't without a function, normally you just define a function that returns the new type, for instance:

newtype MyData = MyData Bool

myData :: String -> MyData
myData "false" = MyData False
myData "true" = MyData True
-- Need to decide how to handle invalid arguments

Now instead of writing MyData "false" you write myData "false" .

Answer 2

You can also use the OverloadedStrings extension in GHC and declare your type to be an instance of IsString :

newtype MyData = MyData Bool deriving (Show)

instance IsString MyData where
    fromString "false" = MyData False
    fromString "true" = MyData True
    fromString "False" = MyData False
    fromString "True" = MyData True

Now you can say something like:

> "false" :: MyData
MyData False
> "True" :: MyData
MyData True

Answer 3

You simply can't.

First, String type has values irrepresentable as Boolean , like "foo" . What do your datatype supposed to return on MyData "foo" ? an error ?

Second, there is the part of haskell philosophy: do everything explicitly. No typecasts, no Foo foo = 1 which calls foo(const int&) like in C++. And that is a good part.

PS: if you get the values from input, just write yourself a proper parser (not just a Read instance - it crushes thread on failure).