Specifying dependencies on Haskell code
apfelmus at quantentunnel.de
Fri May 2 05:27:37 EDT 2008
Duncan Coutts wrote:
> Thomas Schilling wrote:
>> For example, if we write a program that uses the function 'Foo.foo'
>> contained in package 'foo' and we happen to have used 'foo-0.42' for
>> testing of our program. Then, given the knowledge that 'Foo.foo' was
>> introduced in 'foo-0.23' and changed semantics in 'foo-2.0' then we
>> know that 'foo >= 0.23 && < 2.0' is the correct and complete
>> dependency description.
I would go even further and simply use "my program 'bar' compiles with
foo-0.42" as dependency description. In other words, whether the package
foo-0.23 can be used to supply this dependency or not will be determined
when somebody else tries to compile Bar with it.
In both cases, the basic idea is that the library user should *not*
think about library versions, he just uses the one that is in scope on
his system. Figuring out which other versions can be substituted is the
job of the library author. In other words, the burden of proof is
shifted from the user ("will my program compile with foo-1.1?") to the
author ("which versions of my library are compatible?"), where it belongs.
> However it would only help for the development _history_, we still have
> no solution for the problem of packages being renamed (or modules moving
> between packages) breaking other existing packages. Though similarly we
> have no solution to the problem of modules being renamed. Perhaps it's
> just that we have not done much module renaming recently so people don't
> see it as an issue.
With the approach above, it's possible to handle package/module
renaming. For instance, if the package 'foo' is split into 'f-0.1' and
'oo-0.1' at some point, we can still use the union of these two to
fulfill the old dependency 'foo-0.42'.
In other words, the basic model is that a module/package like 'bar' with
a dependency like 'foo-0.42' as just a function that maps a value of the
same type (= export list) as 'foo-0.42' to another value (namely the set
of exports of 'bar'). So, we can compile for instance
bar (f-0.1 `union` oo-0.1)
Of course, the problems are
1) specifying the types of the parameters,
2) automatically choosing good parameters.
For 1), one could use a very detailed import list, but I think that this
feels wrong. I mean, if I have to specify the imports myself, why did I
import foo-0.42 in the first place? Put differently, when I say 'import
Data.Map' I want to import both its implementation and the interface.
So, I argue that the goal is to allow type specifications of the form
'same type as foo-0.42'.
Problem 2) exists because if I have foo-0.5 on in scope on my system and
a package lists foo-0.42 as a dependency, the compiler should somehow
figure out that he can use foo-0.5 as argument. Of course, it will be
tricky/impossible to figure out that f-0.1 `union` oo-0.1 is a valid
So, the task would be to develop a formalism, i.e. some kind of "lambda
calculus for modules" that can handle problems 1) and 2). The formalism
should be simple to understand and use yet powerful, just like our
beloved lambda calculus.
A potential pitfall to any solution is that name and version number
don't identify a compiled package uniquely! For instance,
is very different from
if foo exports the ByteString type. That's the diamond import problem.
In other words, foo-0.3 is always the same function, but the evaluated
results are not.
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