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

It is often the case that using GADTs with phantom types can allow you to constrain which functions can operate on the results of which constructors. I believe this is common practice now in such situations. Nick On Friday, August 31, 2012 09:32:37 PM Paolino wrote:

Hello Timothy

GADTs let you catch more errors at compile time. With them you can give different types to constructors of the same datatype.

regards paolino 2012/8/31

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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 Datum: 31. 8. 2012 Předmět: Re: [Haskell-cafe] Over general types are too easy to make.

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> 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

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on Bars. The best we can do is throw a runtime error when passed a Bar

and

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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.

John

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