Re: [Haskell-cafe] Do expression definition

13 Sep 2010

      On Mon, Sep 13, 2010 at 8:21 AM, Alexander Kotelnikov  wrote:
...
And, also, would it make any difference if
do {p <- e; stmts}      =       let ok p = do {stmts}
   ok _ = fail "..."
 in e >>= ok
is redefined as "e >>= (\p -> do {stmts})"?
This is the magic that allows pattern-match failure in a do expression
to return a normal result. Notice that "fail" and not "error" is
called - each Monad has its own fail method, so that for example:

uncons :: [a] -> Maybe (a, [a])
uncons xs = do { (x:xs) <- return xs; return (x, xs) }

evaluates to Nothing rather than causing an exception when xs is empty.

That this implementation detail ends up in the Monad class is regarded
by many as untidy, though.

Re: [Haskell-cafe] Do expression definition

Ben Millwood