hi
The thing is... it will be a simple parser really. The expressions are already defined and we can't use parsec imports. Below is the types I have. I have a function that removes initial spaces from the start of a string. A function that checks if a substring is part of another string, which use the remove space function also. The next function is: which uses the type:
type Parser = String -> [(Expr,String)] So what does this type really mean? You give it a string, and it will return a list of (Expr, String). I would guess that the list is all
On 7 dec 2007, at 22:55, Ryan Bloor wrote: possible outcomes, and the String in (Expr, String) is probably the rest of the parsed data. If you want to find a correct parse, you probably want to select the first element from that list that has an empty rest. You could write a parser that parses single digits: parseDigit :: String -> [(Int, String)] parseDigit (x:xs) | isDigit x = [read x] Of course you have to define the other cases for parseDigit. If you had a parseMany function that parses many things of the same type, you could combine that with parseDigit to parse natural numbers. The other thing you are really going to need is a choice-operator. In your example, you want to parse terms that are either numbers or "term + term": parseTerm = parseNaturalNumber `parseOr` parseAddition It's probably best to read a good book or tutorial on parsers. There is an excellent textbook on grammars and parsing in Haskell [1], it probably explains exactly what you want. -chris [1] Johan Jeuring, Doaitse Swierstra: Grammars and Parsing: http://www.cs.uu.nl/docs/vakken/gont/diktaat.pdf
readExpression :: String -> Expr
e.g. readExpression "True" = EBool True e.g. readExpression "(23 + 67)" = EAdd (EInt 23) (EInt 67)
------------------------ Types------------------------------------------
data Type = TNone -- badly typed values | TInt -- integer values | TBool -- boolean values deriving Show data Expr = EInt {vInt :: Int} -- integer values | EBool {vBool :: Bool} -- boolean values | EAdd Expr Expr -- (e1 + e2) | EMin Expr Expr -- (e1 - e2) | EMul Expr Expr -- (e1 * e2) | EAnd Expr Expr -- (e1 && e2) | EOr Expr Expr -- (e1 || e2) | ENot Expr -- not e1 | EComp Expr Expr -- (e1 == e2) | ETest Expr Expr Expr -- if e1 then e2 else e3 | ENone -- badly formed expressions
CC: haskell-cafe@haskell.org From: chris@eidhof.nl To: ryanbloor@hotmail.com Subject: Re: [Haskell-cafe] parser Date: Fri, 7 Dec 2007 22:17:54 +0100
Can anyone advise me on how to check whether a string contains ints, chars, bools, etc....
"2345 + 6767" shoudl give IntAdd (2345) (6767) "2345" should give IntT 2345 You need to write a parser. There are a lot of libraries that will help you write a parser. One library that is often used for writing
On 6 dec 2007, at 18:06, Ryan Bloor wrote: parsers in Haskell is called Parsec [1]. There's good documentation on that site on how to use it. Parsec is already included in you distribution. Good luck!
-chris
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