I always define Functor instances, so fmap is already covered, leaving me with a simpler join vs a more complicated bind (comparing complexity of interface, specification and implementation).

  - Conal

On Sun, Jan 9, 2011 at 6:16 PM, Iavor Diatchki <iavor.diatchki@gmail.com> wrote:
Hello,
In my experience, defining monads in terms of "fmap" and "join" leads to code duplication.  The examples we have seen in this thread---so far---are a bit misleading because they compare a partial implementation of a monad (join without fmap) with a complete implementation (bind).  Here is an example of what I mean:

data SP a               = PutChar Char (SP a)
                        | GetChar (Char -> SP a)
                        | Return a

fmapSP :: (a -> b) -> (SP a -> SP b)
fmapSP f (PutChar c sp) = PutChar c (fmapSP f sp)
fmapSP f (GetChar k)    = GetChar (\c -> fmapSP f (k c))
fmapSP f (Return a)     = Return (f a)

joinSP :: SP (SP a) -> SP a
joinSP (PutChar c sp)   = PutChar c (joinSP sp)
joinSP (GetChar k)      = GetChar (\c -> joinSP (k c))
joinSP (Return sp)      = sp

bindSP :: (a -> SP b) -> (SP a -> SP b)
bindSP f (PutChar c sp) = PutChar c (bindSP f sp)
bindSP f (GetChar k)    = GetChar (\c -> bindSP f (k c))
bindSP f (Return a)     = f a

I chose this example because I think that it illustrates nicely how the three operators work, I hope that other readers find it useful.

2011/1/9 Conal Elliott <conal@conal.net>
* The familiarity advantage of (>>=) is a historical accident. I like to see the language improve over time, rather than accumulate accidents.

I would be surprised if choosing ">>=" was an accident: it seems more likely that it was chosen because it matches a commonly occurring pattern in functional programs, and abstraction is all about giving names to common patterns.  I completely agree with the sentiment of your second sentence but I think that adding "join" to the Monad class would be an example of "accumulating an accident" rather then simplifying things.

-Iavor