[Haskell-cafe] exceptions vs. Either
fjh007 at galois.com
Tue Aug 3 19:30:54 EDT 2004
On 03-Aug-2004, Alastair Reid <alastair at reid-consulting-uk.ltd.uk> wrote:
> Another approach is to use a function:
> inContext :: String -> a -> a
> (implemented using mapException) like this:
> inContext "evaluating expression" (eval env e)
> to transform an exception of the form:
> error "Division by zero"
> error "Division by zero while evaluating expression"
As written, that won't work properly; you will get misleading error
messages, due to lazy evaluation. If a division by zero exception is
thrown, it may have occurred when computing the expression e, rather
than in the function eval itself. Furthermore, if eval returns a compound
type, this code won't catch all division by zero errors that occur while
computing eval, only those that occur while computing the top-most node.
To make it work properly, you can do something like
class hyperSeq a where
hyperSeq :: a -> b -> b
inContext :: (hyperSeq a, hyperSeq b) => String -> (a -> b) -> a -> b
and then inContext can force full evaluation of the input and output,
making sure to force full evaluation of the input _before_
inContext msg fn input =
hyperSeq input $
mapException (...) $
let output = fn input
in hyperSeq output output
Of course then you rather completely lose lazy evaluation.
But who wants lazy evaluation, anyway? :)
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Galois Connections, Inc. | of excellence is a lethal habit"
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