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Creating a Random Expression in Haskell

Posted 12-22-2009 at 07:10 AM by primenu

I have a Haskell program for parsing and evaluating an mathematical expression of type:
data Expr
= Num Double
| Add Expr Expr
| Mul Expr Expr
| Sin Expr
| Cos Expr
| Var Name
| Neg Expr
deriving (Eq,Show)

type Name = String

I have the defined a property to perform quickcheck: to check the readExpr and showExpr are valid bothway round, here readExpr takes a string of type "2*3+x" and converts it to Expression of type Expr and vice versa for showExpr
prop_showReadExpr :: Expr -> Bool
prop_showReadExpr ex = case (readExpr(showExpr ex)) of
Nothing -> False
_ -> showExpr(fromJust(readExpr(showExpr ex))) == showExpr ex

instance Arbitrary Expr where
arbitrary = sized arbExpr

arbExpr :: Int -> Gen Expr
arbExpr s =
[ (1, do n <- arbitrary return (Num (abs(n))))
, (s, do a <- arbExpr s' b <- arbExpr s' return (Add a b))
, (s, do a <- arbExpr s' b <- arbExpr s' return (Mul a b))
, (s, do a <- arbExpr s' return (Sin (a)))
, (s, do a <- arbExpr s' return (Cos (a)))
, (1, do return (Var "x"))
where s' = s `div` 2

Now I want to get a random expression fro my GUI Calculator, where whenever the user presses a button and the Calculator generates a random Expression of the above type.Can I use the arbExpr which returns a Expression of type 'Gen Expr' for my GUI Calculator, the problem I am having is whenever I try to use :showExpr (arbExpr 9) to get an Expr I get type miss match error saying:
Couldn't match expected type `Expr'
against inferred type `Gen Expr'
In the first argument of `showExpr', namely `(arbExpr 1)'
In the expression: showExpr (arbExpr 1)
In the definition of `it': it = showExpr (arbExpr 1)

Thanks in Advance..
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