Abstract data types

[Keith Wansbrough]

> I have attached our module for the abstract data type Stack
> (we follow the Haskell 98 report and don't want to go into
> 1.4 compatibility). We expected the representation of stacks
> were hidden but Hugs let us write an expression with the
> "hidden" constructor StackC:
> 
> Stack> :l Stacks
> Reading file "Stacks.hs":
> Parsing....................
> Dependency analysis........
> Type checking..............
> Compiling..................
> 
> Hugs session for:
> /opt/lib/hugs/lib/Prelude.hs
> Stacks.hs
> Stacks> StackC [1]
> StackC [1]
> Stacks> 
> 
> This problem occurs only in the top level, if we program a
> client module importing the module Stacks and we try to
> write a expression using directly StackC then Hughs gives an
> error message (great!).
> 
> This is a bug in the top level, isn't it?

This is not a bug, it's a Feature[TM]!  Notice the prompt says "Stacks>".  This means that you are "in" the module Stacks, and so can see everything visible to a top-level function in the module Stacks.

If you write another file, say Main.hs, containing just the single line "import Stacks", then do ":l Main", you will find
Hugs session for:
/usr/groups/haskell/lib/hugs/lib/Prelude.hs
Stacks.hs
Main.hs
Type :? for help
Main> StackC [1]
ERROR: Undefined constructor function "StackC"
Main> 

which I think is what you wanted.

HTH.

--KW 8-)


-- 
Keith Wansbrough <kw217@cl.cam.ac.uk>
http://www.cl.cam.ac.uk/users/kw217/
Cambridge University Computer Laboratory.