Re: [Haskell-cafe] IO (Either a Error) question

On Sat, May 8, 2010 at 1:16 AM, Ivan Lazar Miljenovic wrote:

David Menendez writes:

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On Sat, May 8, 2010 at 12:15 AM, Ivan Lazar Miljenovic

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Well, any time you have a do-block like this you're using failable patterns:

maybeAdd       :: Maybe Int -> Maybe Int -> Maybe Int maybeAdd mx my = do x <- mx                    y <- my                    return $ x + y

This is true in the sense that the translation for the do syntax in the Haskell report uses fail.

do { p <- e; stmts } =     let ok p = do { stmts }         ok _ = fail "..."     in e >>= ok

However, it's also true that the fails introduced by the translation of maybeAdd will never be invoked, since the two patterns are irrefutable.

Huh?  What about "maybeAdd (Just 2) Nothing" ?

That does not invoke fail. Let's take a simpler example: do { x <- Nothing; stmt }. This translates to let ok x = do { stmt } ok _ = fail "..." in Nothing >>= ok By the definition of (>>=) for Maybe, 'ok' is never called.

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That is, maybeAdd would work exactly the same if the do syntax translation were changed to read:

do { p <- e; stmts } = e >>= \p -> do { stmts }

Wait, are you using "irrefutable" as "it will still work if we make do blocks work the way I want"?

I am using "irrefutable" to refer to patterns which always match. From the Haskell Report, section 3.17.2:

It is sometimes helpful to distinguish two kinds of patterns. Matching an irrefutable pattern is non-strict: the pattern matches even if the value to be matched is _|_. Matching a refutable pattern is strict: if the value to be matched is _|_ the match diverges. The irrefutable patterns are as follows: a variable, a wildcard, N apat where N is a constructor defined by newtype and apat is irrefutable (see Section 4.2.3), var@apat where apat is irrefutable, or of the form ~apat (whether or not apat is irrefutable). All other patterns are refutable.

-- Dave Menendez http://www.eyrie.org/~zednenem/