[Hat] comments on hat 2.02

[Malcolm Wallace]

Bernard James POPE <bjpop at cs.mu.OZ.AU> writes:

> I've been playing with hat 2.02 installed via a debian package.
> Using ghc 6.02 (I think) to build the transformed code.
> (Note: I couldn't get it to build on my own with ghc 6.02).

Yes, the original tarball of 2.02 needs a bunch of patches to get it
to compile with ghc-6.2.  We hope to release a new version 2.04 in
the relatively near future to solve those issues, and also incorporate
numerous other small bugfixes and enhancements.

If you want to play with the current CVS version, do feel free.  It
is available from cvs.haskell.org.

> I discovered that hat doesn't like it if I define a type called "List",
> as in:
> 
>    data List = List deriving Show
>    main = print List
> 
> When ghc tries to compile the transformed code it finds that 
> my List type clashes with one that's coming via the Hat.Prelude.

Hmmm, yes there are a few types and constructors that fall into
this category.  You are right, they are not currently documented.
They are:

    Fun(Fun)
    List(Cons,List)
    Tuple0(Tuple0) ...  Tuple15(Tuple15)

Interestingly, nhc98 does not complain, even though the usage you
show is indeed technically ambiguous.

> My guess is that all imports of Hat's special libraries should be
> qualified imports.

Yes, and they already are.  The problem in this instance is that
the transformed Hat.Prelude is re-exporting Fun, List, Tuple0, etc,
when there is no need (they will always be available qualified from
elsewhere anyway).  Unfortunately, the untransformed equivalents
([],(,),(->)) which give rise to them /do/ need to appear in the
export list of the untransformed Prelude.  So there is no immediate
and easy solution to this issue.

> Another problem I have encounterd is that the trace tools 
> exit leaving my terminal in a bad state, namely my terminal does
> not echo its output any more. I have to type "reset" at the
> prompt to get it back. This is on Debian linux with the gnome
> terminal.

We also use linux (Slackware) with the gnome-terminal.  As far as I
am aware, the terminal should be left OK provided the browser exited
normally.  If the browser crashes for any reason, it can leave the
terminal settings badly.  Are you exiting with a :quit command?

Hat-trail issues
    stty -icanon min 1 -echo
on entry and
    stty icanon echo
to reset on exit.

> A small gripe is with the extra files that hat puts in the current
> working directory. My preference would be to put everything in
> the Hat subdirectory. Perhaps there is an option to do this?

Do you mean the .hat, .hat.output, and .hat.bridge files?  We could
add an option to place them in another directory I suppose.

> A more general comment is with the display of functional values.
> The item:
>       1.  option to show lambda expressions in full
> appears under the heading "Controversial Wishes".
> I wonder what it means to be controversial in this case?

The controversy (purely between the developers, of course) is whether
it is useful to show anything in place of the lambda (\...) symbol.
What would you show - the source code? a mapping?

  * In principle, given a source reference attached to the lambda
    in the trace itself, the browser could "snip out" the full
    original source expression and insert it into the display.
    (We currently only have the source start position, not the full
    extent of the expression, but we hope to fix that eventually.)

    But the source code display is already available! (by using a
    :source command to open a window with the relevant expression
    highlighted).

  * You suggest an "observe"-like mapping as an extensional
    representation of the anonymous function.  For one thing, such
    a mapping could be extremely large.  And for another, it should
    in principle already be available, by a direct link to hat-observe.

    The fact that there is currently no actual mechanism to pass
    the particular lambda to hat-observe is something we would
    definitely like to fix.  But the controversy would be about
    whether to encourage the user to jump between browsers, or to
    "in-line" the results of hat-observe into the display of another
    browser instance.

> One last question is will hat-detect be released again?

Yes, it will.  An incomplete version of hat-detect is currently
sitting in CVS, but it still requires some work to its user-interface
before it can be released properly.

There is also the prospect that we might release some new browsers,
based on work by undergraduates.  hat-anim animates the reduction
of expressions, and black-hat detects the program cycle leading to
a black hole.

Regards,
    Malcolm