broken Monad Either instance?

[Maciej Marcin Piechotka]

On Mon, 2011-11-28 at 20:47 +0100, Bas van Dijk wrote:
> On 28 November 2011 20:22, Jake McArthur <jake.mcarthur at gmail.com> wrote:
> > I'd just like to add one further remark. If we had a separate
> > MonadFail class then we could get back the original behavior by
> > imposing an Error constraint on it, but without the problems we get by
> > imposing it on the Monad instance instead.
> 
> The problem with a separate MonadFail class is when to add that
> constraint to the type of a do-expression.
> 
> Currently do-expression with a bind:
> 
> do {p <- e; stmts}
> 
> are translated to:
> 
> let ok p = do {stmts}
>     ok _ = fail "..."
> in e >>= ok
> 
> With a separate MonadFail class we have 4 options:
> 
> 1) Simply translate to: "e >>= \p -> do{stmts}" instead. This means
> pattern match errors are always turned into errors.
> 

It may be just me but I always desugared in head
do {p <- e; stmts} => e >>= \p -> do {stmts}

Hence I would prefer the option 1) with special syntax for MonadFail:

do {p <?- e; stmts} =>
  e >>= \p' -> case p' of p -> do {stmts}; _ -> fail "..."

> 2) Give all do-expressions a MonadFail constraint.
> 
> 3) Only give do-expressions with pattern bindings a MonadFail constraint.
> 
> 4) Only give do-expressions with pattern bindings with uncomplete
> patterns a MonadFail constraint.
> 
> To me 2, 3 and 4 seem fishy. I like 1 a lot but I'm unsure how many
> programs will break because of it.
> 

From me +1 for 1) with the same note.

> 
> Regards,
> 
> Bas

Regards

-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 836 bytes
Desc: This is a digitally signed message part
URL: <http://www.haskell.org/pipermail/libraries/attachments/20111201/671b452d/attachment.pgp>