Brian Huffman wrote:
Hello all,
In Haskell, when I declare the following type:
data Foo = MkFoo (Int, Bool)
the constructor "MkFoo" has a run-time cost: A pointer must be
dereferenced whenever I pattern-match on MkFoo, and memory is
allocated when I apply the MkFoo constructor.
On the other hand, if I declare this type:
newtype Foo = MkFoo (Int, Bool)
then the MkFoo constructor has no existence at run-time: Types Foo and
(Int, Bool) have the same representation on the heap, and the MkFoo
constructor function is a no-op.
Here's my question: When I declare a single-constructor datatype in
Poly/ML, is it more like Haskell's "data" or "newtype"?
In Poly/ML a single constructor is the identity function so the answer
is presumably that it's like "newtype" (I don't know Haskell).
Here's the reason I'm asking: I'm thinking of using single-constructor
datatypes to implement abstract types, by defining the datatype within
the module, but not including the constructor function in the module
signature. (This is exactly how people usually do abstract types in
Haskell - just don't export the constructors.) I'd like to know if
there are any pros or cons of this method compared to using abstype.
There's no difference in implementation between an abstype and a
datatype. "abstype" is a very old construction in Standard ML and
predates the modules system let alone the opaque signature matching of
ML 97. It's probably better on stylistic grounds to use signature
matching to hide the constructors but I don't think there's any
difference in the underlying implementation.
David
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