[Haskell-cafe] What's up with this Haskell runtime error message:
Michael Goodrich
michael.goodrich at gmail.com
Wed Apr 5 16:51:10 EDT 2006
Oops, I just realized that you gave me the answer, namely that it won't find
fixed points of numeric sets of equations.
Pity, that would really have made Haskell useful for this kind of scientific
computing.
On 4/5/06, Brandon Moore <brandonm at yahoo-inc.com> wrote:
>
> Michael Goodrich wrote:
> > Looks like my calulation involves a self referential set of definitions.
> >
> > Is Haskell not able to deal with a self referential set of definitions?
> >
> > I was frankly hoing it would since otherwise there is then the specter
> > of sequence, i.e. that I have to finesse the order in which things are
> > calculated so as to avoid it.
> >
> > Thoughts?
>
> Lazy evaluation is great with self-referential definitions, but id
> doesn't do so well with ill-founded definitions. It also won't find
> fixpoints of numeric equations. Here are some examples, and then some
> explanation.
>
> Things that work:
>
> {- for interactive use in ghci -}
> let ones = 1:ones
> --infinite list of ones
> let counting = 1:map (+1) counting
> -- infinite list counting up from one
> let fibs = 1:1:zipWith (+) fibs (tail fibs)
> --fibbonacci numbers
>
> {- A larger program.
> turns references by name into direct references
> Try on a cyclic graph, like
> buildGraph [("a",["b"]),("b",["a"])]
> -}
> import Data.List
> import Data.Map as Map
>
> data Node = Node String [Node]
> type NodeDesc = (String, [String])
>
> buildNode :: Map String Node -> NodeDesc -> Node
> buildNode env (name,outlinks) =
> Node name (concat [Map.lookup other finalBinds | other <- outlinks])
>
> buildGraph :: [(String,[String])] -> [Node]
> buildGraph descs = nodes
> where (finalBinds, nodes) = mapAccumR buildExtend Map.empty descs
> buildExtend binds desc@(name,_) =
> let node = buildNode finalBinds desc
> in (Map.insert name node binds, node)
>
>
> Things that will not work:
>
> let x = x
> -- no information on how to define x
>
> let x = 2*x + 1
> -- this is not treated algebraically
>
> let broke = 1:zipWith (+) broke (tail broke)
> -- the second element depends on itself
>
>
> Recursive definitions in Haskell can be explained by
> saying that they find the least-defined fixedpoint of the equations.
> Every type in Haskell has all the usual values you would have in a
> strict language, plus an undefined value which corresponds to a
> nonterminating computation. Also, there are values where subterms
> of different types are undefined values of that type rather.
>
> For example, with pairs of numbers there are these posibilites
> (x,y)
> / \
> (_|_,x) (x,|_|)
> \ /
> (_|_,_|_)
> |
> _|_
> where x and y represent any defined number, and _|_ is "undefined",
> or a non-terminating computation. A value on any line is
> considered more defined than values on lower lines. Any value which can
> be obtained from another by replacing subterms with _|_ is less defined,
> if neither can be made from the other that way than neither is more
> defined that the other.
>
>
> Think of a definition like x = f x. That will make x the least-defined
> value which is a fixedpoint of f. For example, numeric operations are
> (generally) strict, so _|_ * x = _|_, _|_ + x = _|_, and
> _|_ is a fixedpoint of \x -> 2*x + 1.
>
> for broke, consider the function f = \l -> 1:(zipWith (+) l (tail l))
> f (x:_|_) = 1:zipWith (+) (1:_|_) (tail (1:_|_))
> = 1:zipWith (+) (1:_|_) _|_
> = 1:_|_
> so 1:_|_ is a fixedpoint. It's also the least fixedpoint, because
> _|_:_|_ is not a fixedpoint, and
> f _|_ = 1:<something>, so _|_ is not a fixedpoint either. If I try that
> definition of broke, ghci prints "[1" and hangs, indicating that the
> rest of the list is undefined.
>
> If multiple definitions are involved, think of a function on a tuple of
> all the definitions:
>
> x = y
> y = 1:x
>
> corresponds to the least fixedpoint of (\(x,y) -> (y,1:x))
>
> The recursiveness in the graph example is more tedious to analyze like
> this, but it works out the same way - whatever value of "finalBinds" is
> fed into the recursive equation, you get out a map built by taking the
> empty map and adding a binding for each node name. Chase it around a few
> more times, and you'll get some detail about the nodes.
>
> Also, posting code really helps if you want specific advice. Thanks to
> the hard work of compiler writers, the error message are usually precise
> enough for a message like this to describe the possibilites. If you
> enjoy my rambling I suppose you should keep posting error messages :)
>
> Brandon
>
> > cheers,
> >
> > -Mike
> >
> >
> > ------------------------------------------------------------------------
> >
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>
>
>
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