[Haskell-cafe] OOP'er with (hopefully) trivial questions.....

[Wolfgang Jeltsch]

Am Montag, 17. Dezember 2007 13:04 schrieb Jed Brown:
> […]

> When your type only has one constructor, newtype is preferred over data, but
> they are semantically equivalent.

They are *not* semantically equivalent, as has already been said more or less.  
data adds an extra level of indirection.  With 

    data A a = MkA a,

_|_ (i.e., undefinedness) is different from MkA _|_.  If I don’t know anything 
about a value of A a then this is not the same as knowing that the value is 
at least an application of MkA.

newtype just creates wrapper types and it’s very unfortunate that it uses 
syntax similar to data because it’s very different.  With

    newtype A a = MkA a,

you just create a wrapper type A a for each type a.  Applying the constructor 
just means casting from a to A a, and pattern matching just means casting 
from A a to a.  Type-casting _|_ yields botton, that’s why MkA _|_ is _|_ and 
pattern matching _|_ against A x doesn’t fail but assigns _|_ to x.

> […]

Best wishes,
Wolfgang