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

John Wiegley johnw at newartisans.com
Fri Aug 31 20:47:22 CEST 2012

>>>>>   <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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