[Haskell-beginners] Hutton Ex 8.9.5 & Ex 8.9.6

[David McBride]

Your answers are good.  toEnum is redundant.  Reconstructing the Add is
unnecessary.  His second function is fine but it doesn't use folde.

I would take hlint's suggestion and use const.  This situation is a common
use case.

On Fri, Sep 14, 2018 at 4:04 AM trent shipley <trent.shipley at gmail.com>
wrote:

> Pankaj Godbole's code examples for 8.9.6 are sufficiently more complex
> than mine,that I am worried I missed an obvious bug I'm not experience
> enough to know about or test for.
>
> Correction or reassurances would be appreciated.
>
> Regards,
>
> Trent
>
> {--
>
> 5. Given the type declaration
>
> data Expr = Val Int | Add Expr Expr
>
> define a higher-order function
>
> folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a
>
> such that folde f g replaces each Val constructor in an expression by the
> function f, and each Add constructor by the function g.
>
> Hutton, Graham. Programming in Haskell (Kindle Locations 3364-3371).
> Cambridge University Press. Kindle Edition.
>
> --}
>
> data Expr = Val Int | Add Expr Expr
>
> -- Trent's stuff
>
> folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a
> folde f g (Val x) = f x
> folde f g (Add left right) = g (folde f g left) (folde f g right)
>
> {--
>
> 6. Using folde, define a function
>
> eval :: Expr -> Int
>
> that evaluates an expression to an integer value, and a function
>
> size :: Expr -> Int
>
> that calculates the number of values in an expression.
>
> Hutton, Graham. Programming in Haskell (Kindle Locations 3372-3374).
> Cambridge University Press. Kindle Edition.
>
> --}
>
> -- Trent's stuff
>
> eval :: Expr -> Int
> eval = folde id (+)
>
> size :: Expr -> Int
> size = folde (\_ -> 1) (+) -- hlint suggests \_ as const 1
>
> test0 :: Expr
> test0 = Val 1
>
> test1 :: Expr
> test1 = Add (Val 1) (Val 2)
>
> test3 :: Expr
> test3 = Add (Val 1) (Add (Val 2) (Val 3))
>
> -- the below are from:
> -- Pankaj Godbole
> -- https://github.com/pankajgodbole/hutton/blob/master/exercises.hs
>
> evalE             :: Expr -> Int
> evalE (Val n)     =  n
> evalE (Add e1 e2) =  folde toEnum (+) (Add e1 e2)
> -- doesn't a Val have to prefix an int?  Then why is f "toEnum"?
>
>
> numVals             :: Expr -> Int
> numVals (Val n)     =  1
> numVals (Add e1 e2) =  numVals e1 + numVals e2
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