[Haskell] Re: Mixing monadic and non-monadic functions
claus.reinke at talk21.com
Fri Sep 9 19:55:15 EDT 2005
life is funny, isn't it? so many people so eagerly discussing conversion
between non-monadic and monadic code, yet when we asked for your
opinions and suggestions on this very topic only a short while ago,
we got a total of 4 (four) replies - all quite useful, mind you, so we were
grateful, but still one wonders.. we might have assumed that not many
people cared after all:
shall I assume that all participants in this discussion have joined
the Haskell parade since then, and have proceeded rapidly to the
problems of monadic lifting?-) in which case I'd invite you to have
a look at that survey and the papers mentioned.
> > I thought the easy answer would be to inject non-monadic values into the
> > monad (assuming one already rejiggered things to do automatic lifting).
I'd phrase it slightly differently: what (I think) one wants are implicit coercions
between monadic and non-monadic types of expressions, where the coercions
lift non-monadic values into the monad in question, while embedding monadic
computations in the current monad to get a non-monadic result if only that is
needed (although one might think of the latter as partially lifting the operation
that needs the non-monadic result).
only I wouldn't want those implicit coercions to be introduced unless
programmers explicitly ask for that (one usually only converts code from
non-monadic to monadic once, and while the details of that step might
be tiresome and in need of tool-support, the step itself should be explicit
- see my comment on (2) below).
> Note that in (a), "pure" values are never used where monads are asked
> for, only the other way around.
that is probably where some would beg to differ - if you lift operations,
why not lift values as well?
> I think that supporting syntax (a) for semantics (b) should be a
> feature because: (1) it is (usually) obvious what (a) means; (2) it
> eliminates the single-use variable 'v' - single-use variables like
> this occur a lot in monadic Haskell code, and I think they make it
> harder to read and write; (3) it would support the math-like syntax
> that I presented in my original message.
(1) "(usually) obvious" is tech-speak for "(perhaps) possible to
figure out, though probably not uniquely determined"?-)
when mathematicians abuse notation in the "obvious" way,
there is usually an assumed context in which the intended
abuses are clearly defined (if not, there is another context
in which the "obvious" things will go unexpectedly awry).
(2) the nice thing about Haskell is that it *distinguishes* between
monadic and non-monadic computations, and between evaluation
and execution of monadic computations. if you want everything
mixed into one soup, ML might be your language of choice
(http://portal.acm.org/citation.cfm?id=178047 , if I recall correctly?
see the paper discussed in
for one application that demonstrates the power/danger of such
(3) using math-like syntax for lifted expressions is common practice
in some families of Haskell-DSELs, eg. Conal Elliot's Fran. As John
pointed out, the predefined class-hierarchy is not really helpful
for such endeavours, but if one isn't picky, one may ignore classes
not used.. the "trick" is to lift even constants, so when you get
to applications, all components are already lifted, and lifting most
arithmetic works out fine (Boolean operations are another matter).
note, however, that the resulting language, while looking
mathematically pure and permitting concise expression of complex
circumstances, may not have the reasoning properties you expect..
> It might be hard to modify the type checker to get it to work, but I
> think it is possible, and I see no reason not to be as general as
here I'd agree, although in contrast to you, I'd be talking about a
complex refactoring under programmer control, not about an implicitly
invoked collection of coercions. I played with that idea after Martin
Erwig visited our refactoring project in March, and got to a prototype
type-coercion inference system for a very simple functional language,
because I found the situation with various existing and, as Erwig/Ren
pointed out, apparently unrelated monadification algorithms confusing.
apart from the various styles of monadification, which we'd like
to permit, and have the programmer select, e.g., by type annotations,
there is the slight problem that there are an unbounded number of
different monadifications (more if one wants to keep annotations to
a minimum), so one needs a sensible bound (one that does not
exclude any of the alternatives one might want). one also might
want to be able to choose between the alternatives (or tune the
system so that taking the first choice works out ok most of the
time). oh, and it shouldn't be too inefficient, and it is really a pain
to re-implement a type-system just to add a few coercion rules to
it (which is why I haven't extended my mini fpl to Haskell yet..).
in light of this, perhaps some more participants in this discussion
might want to look into contributing their suggestions to our old
More information about the Haskell