[Haskell-cafe] Over general types are too easy to make.

timothyhobbs at seznam.cz timothyhobbs at seznam.cz
Fri Aug 31 20:58:27 CEST 2012

Sure, but that's relying on the promise that you're passing it a valid 
BadFrog...  Consider then:

deBadFrog $ BadFrogType (BadBar { badFoo = 1})

---------- Původní zpráva ----------
Od: John Wiegley <johnw at newartisans.com>
Datum: 31. 8. 2012
Předmět: Re: [Haskell-cafe] Over general types are too easy to make.
">>>>> <timothyhobbs at seznam.cz> writes:

>> data BadFoo =
>> BadBar{
>> badFoo::Int} |
>> BadFrog{
>> badFrog::String,
>> badChicken::Int}

> This is fine, until we want to write a function that acts on Frogs but not
> on Bars. The best we can do is throw a runtime error when passed a Bar and
> not a Foo:

You can use wrapper types to solve this:

data BadBarType = BadBarType BadFoo
data BadFrogType = BadFrogType BadFoo

Now you can have:

deBadFrog :: BadFrogType -> String

And call it as:

deBadFrog $ BadFrogType (BadFrog { badFrog = "Hey", badChicken = 1})

Needless to say, you will have to create helper functions for creating Bars
and Frogs, and not allow your BadBar or BadFrog value constructors to be
visible outside your module.


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