Re: [Haskell-cafe] All-fail case in asum

Of course, but I want the *perfect* solution. I think, this Parser type is not perfectly monoidal, so in general asum and (<|>) are not interchangeable. On 10 Jun 2016 19:51, "Erik Hesselink" wrote:

Disregarding is `asum` should be changed, perhaps you could fix your issue by doing:

myParser = asum [tryA, tryB] <|> fail "input must be either A or B"

Erik

On 10 June 2016 at 16:37, Юрий Сыровецкий (Yuriy Syrovetskiy) wrote:

...

Hello

I want to define some parser such way:

myParser = tryA <|> tryB <|> fail "input must be either A or B"

It works. But then I want to rewrite it with asum:

myParser = asum [tryA, tryB, fail "must be A or B"]

It works, but the wrong way. Instead of my error it writes "empty". Just "empty".

It is so because in base library

asum = foldr (<|>) empty

What if it was defined

asum [] = empty asum [x:xs] = x <|> asum xs

It would help me with my parser. But what can this break? Why isn't this done yet?

-- Yuriy Syrovetskiy, http://cblp.su _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe