Re: [Haskell-beginners] Comfortable handling of module hierarchies

Do you know any document pointing out the rationale behind this decision about modules taken by the Haskell designers? Thanks. -Damodar On Sat, Sep 15, 2012 at 9:30 PM, Karl Voelker wrote:

On Fri, Sep 14, 2012 at 6:23 PM, Christopher Howard wrote:

...

import qualified Plant as P

P.Fruit.Raspberry.jam

Short answer: P.Fruit.Raspberry.jam would work if you said:

import qualified Plant.Fruit.Raspberry as P.Fruit.Raspberry

Long answer:

You can't have exactly what you want because the Haskell module namespace isn't exactly heirarchical. Here's an excerpt from the Haskell 98 report [1]:

"The name-space for modules themselves is flat, with each module being associated with a unique module name (which are Haskell identifiers beginning with a capital letter; i.e. modid)."

Notice that this doesn't allow for dots in module names. A commonly-provided language extension allowed dots in module names, and compilers took these dots as a signal to look for a module's source at a particular place in the directory tree, but the semantics of the language didn't have a heirarchy of modules.

Things haven't changed much in Haskell 2010, other than the existing use of dots being formalized [2]:

"Module names can be thought of as being arranged in a hierarchy in which appending a new component creates a child of the original module name. For example, the module Control.Monad.ST is a child of the Control.Monad sub-hierarchy. This is purely a convention, however, and not part of the language definition; in this report a modid is treated as a single identifier occupying a flat namespace."

In your code snippet, P.Fruit.Raspberry doesn't work because although P refers to the same module as Plant, there isn't anything "inside" P (or Plant) called Fruit.Raspberry.

-Karl

[1] http://www.haskell.org/onlinereport/modules.html [2] http://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-980005

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners