For anyone else benefit, this is how I fixed issue. 1. download Parsing.lhs from author's website. 2. Also download parser.lhs and paste contents into my prog_haskell.hs file, removing text comments and > symbols. 3. At top of my prog_haskell.hs file you need import Parsing You can then call the functions in WinGHCi, eg eval "3+5*2" For some reason I was not able to work out how to import both Parsing.lhs and parser.lhs together. Will have to read up on modules later. On 7 January 2014 11:43, Christian Maeder wrote:

Hi Angus,

did you get an answer for your question? (Just pressing "reply" only answers to the list, as happened to my earlier answer to your initial question.)

I don't know if WinGHCi works like ghci under linux (and I don't know which files Parsing.lhs and parser.lhs you tried to load.)

At the ghci Prompt you can type ":browse" to see which names are in scope. Maybe parser.lhs could not be loaded for some reason that I cannot reproduce. (Maybe eval is explicitly not exported?)

Cheers Christian

Am 03.01.2014 18:43, schrieb Angus Comber:

...

I found some notes on the authors website explaining that the book code would no longer work. So I downloaded two files: Parsing.lhs and parser.lhs. In WinGHCi I then issued :load on both files.

parser.lhs contains a function, eval, as follows:

<other stuff above>

...

eval :: String -> Int eval xs = case (parse expr xs) of [(n,[])] -> n [(_,out)] -> error ("unused input " ++ out) [] -> error "invalid input"

But in WinGHCi if I run: eval "2*3+4" I get error := Not in scope: `eval'

I assume this is some sort of namespaces feature of Haskell that I have not read about? Or I somehow have to state what library modules to use??? Can anyone help?

On 3 January 2014 15:58, Angus Comber mailto:anguscomber@gmail.com> wrote:

I am reading Chapter 8 of Programming Haskell by Graham Hutton and trying to run the code in the book.

It seems that there are already defined library implementations of Parser so I used Parser' and generally ' on the end of each identifier to attempt to eliminate this problem.

So the code I copied from the book is:

type Parser' a = String -> [(a, String)]

return' :: a -> Parser' a return' v = \x -> [(v, x)]

failure' :: Parser' a failure' = \inp -> []

-- item doesn't seem to conflict with anything so no ' item :: Parser' Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)]

parse' :: Parser' a -> String -> [(a, String)] parse' p inp = p inp

p :: Parser' (Char, Char) p = do x <- item item y <- item return' (x,y)

When run from WinGHCi I get error:

prog_haskell.hs:458:9: Couldn't match type `[(Char, String)]' with `Char' Expected type: String -> [((Char, Char), String)] Actual type: Parser' ([(Char, String)], [(Char, String)]) In the return type of a call of return' In a stmt of a 'do' block: return' (x, y) In the expression: do { x <- item; item; y <- item; return' (x, y) }

458.9 is line at end with return' (x,y)

Also in the chapter was the definition of >>= sequencing. as in:

(>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v,out)] -> parse (f v) out

But I assumed this would already be in standard Haskell library and so I didn't need to define. But even when I did add, still get same error.

How do I fix this?

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