Got it. Thanks! Michael --- On Mon, 6/1/09, Daniel Fischer wrote: From: Daniel Fischer Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: haskell-cafe@haskell.org Date: Monday, June 1, 2009, 1:51 PM Am Montag 01 Juni 2009 19:02:36 schrieb michael rice:

All good so far, but then tried to convert Failable from Computation to Monad

instance Monad Failable where return = Success fail = Fail >>= (Success x) f = f x >>= (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x

and got the following error.

Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted )

graph5.hs:34:4: parse error on input `>>=' Failed, modules loaded: none. Prelude>

When you use an operator in prefix position, you must enclose it in parentheses, like you must enclose a function in backticks if you use it infix. So the definition of (>>=) should read     (>>=) (Success x) f = f x     (>>=) (Fail s) _ = Fail s or, defining it in infix position,     (Success x) >>= f = f x     (Fail s) >>= _ = Fail s

Complete code follows.

Michael

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