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haskellcustom-data-type
matt
matt

Reputation: 2039

Haskell: functions in custom data types

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

Upvotes: 0

Views: 1497

Answers (3)

Heimdell
Heimdell

Reputation: 647

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.

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

Upvotes: 0

chepner
chepner

Reputation: 531165

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

Upvotes: 0

Guvante
Guvante

Reputation: 19203

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".

Upvotes: 6

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