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Today's Topics:
1. Hutton Ex 8.9.5 & Ex 8.9.6 (trent shipley)
2. Re: Hutton Ex 8.9.5 & Ex 8.9.6 (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Fri, 14 Sep 2018 01:04:12 -0700
From: trent shipley <trent.ship...@gmail.com>
To: Haskell Beginners <beginners@haskell.org>
Subject: [Haskell-beginners] Hutton Ex 8.9.5 & Ex 8.9.6
Message-ID:
<caeflybkksnw-jcwms3fzfc7taumsdbexko9guezptj+zqki...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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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Message: 2
Date: Fri, 14 Sep 2018 08:06:02 -0400
From: David McBride <toa...@gmail.com>
To: Haskell Beginners <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Hutton Ex 8.9.5 & Ex 8.9.6
Message-ID:
<CAN+Tr43=odzq---bb8pxnydjvwhp2+dm_oroawue_eunoik...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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.ship...@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
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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