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Compiler feature - useful or not?
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Date: 2007-11-14 (14:35)
From: weis@y...
Subject: Re: [Caml-list] Compiler feature - useful or not?
Hi Alain,

> Pierre Weis wrote:
> >type row = private int;;
> The only difference with an abstract type is that

a private type is not an abstract type but a concrete type.

> some generic operations (comparision, equality) can be optimized, and
> currently, it happens only for some basic types. So exporting a private
> abbreviation (instead of an abstract type) is useless when the type is not
> a basic type. Is it correct?

I don't understand what you mean by ``exporting a private type abbreviation
is useless''. In fact, you cannot create a private abbreviation if you do not
`export'' it and define it as private when exporting, isn't it ?

(Clearly, if you define a private type abbreviation in an implementation
file, this definition is completely useless since you cannot define any
value of the private type).

However, if I interpret what you said as: ``if we only want to optimize the
compilation, there is no point to define a private type instead of an
abstract type in the case of a complex type abbreviation'' then the question
is somewhat equivalent to ask ``in which case the compiler can optimize when
faced to a type abbreviation ?'': the answer is complex but in particular the
compiler can optimize a type t defined as being a (concrete) abbreviation for
a float array or a (concrete) abbreviation for a record of floats.

In any case, the purpose of introducing private abbreviation type definitions
is not to optimize programs but to write clearer and safer programs. In
particular because private data types can ensure invariants the same way as
abstract data types do; and also because private data types support pattern
matching the same way as concrete types do.

Once more, private abbreviation types being concrete, they get all the
advantages of concrete types and all their drawbacks (in particular wrt
abstraction of value representation, evolution of implementation programs,
openness of algorithms, meta informations).

> I find it somewhat disturbing to expose low-level optimization features 
> as part of the type system.

No low-level optimization is ``exposed in the type system'': the low level
(type-based) optimizations are just an automatic side effect of a private
data type being indeed a concrete type. In other words, would you say that
you ``expose low level optimization as part of the type system'' if you ever
define a vector as being a float array instead of being a M.t array value
with M.t being an abstract abbreviation for float ?

In some cases, the answer can be yes: you may need to ``unabstract'' types
for the sake of efficiency. But generally speaking, you just use the type
float because it is the type of values you need, the optimizations come then
from the compiler as a very welcome bonus.

> Couldn't a similar thing (specializing operations on integers) be achieved
> by always storing manifest types in .cmxs files?  (Within a single
> compilation unit, the compiler could keep type definitions across module
> boundaries.)

That's another story: the problem with this approach is to prevent breaking
the separate compilation feature of the actual compiler.

In any case, private type abbreviations have been implemented as a new useful
tool for the programmer, not as a way to optimize programs. Type-based
compiler's optimizations are just an extra plus, I thought it was worth to
mention that private type abbreviations are fully compatible with these

But in the first place, private type abbreviations are useful to define proper
concrete sub types of another type.

All the best,

-- Pierre