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Today's Topics:
1. No ghci return for homemade types (Lawrence Bottorff)
2. Re: No ghci return for homemade types (Francesco Ariis)
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Message: 1
Date: Sun, 3 Jan 2021 16:22:12 -0600
From: Lawrence Bottorff <borg...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] No ghci return for homemade types
Message-ID:
<CAFAhFSXHSyoinnL3tsVpV=tabzacerjwm1ddjvzk0prntyk...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I've tried this
data MyMaybe a = Just a | Nothing
maybeHead :: [t] -> MyMaybe t
maybeHead [] = Nothing
maybeHead (x:_) = Just x
and it evaluates fine. But this
maybeHead [1,2,3]
produces error:
: • No instance for (Show (MyMaybe Integer))
: arising from a use of ‘show’
: • In the expression: show (maybeHead [1, 2, 3])
: In an equation for ‘it’: it = show (maybeHead [1, 2, 3])
Similarly,
data Color a = Blue a | Green a | Red a
myFavoriteColor :: Color Int
myFavoriteColor = Green 50
but now I'm stuck. How can I access the "Green 50" inside myFavoriteColor?
LB
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Message: 2
Date: Sun, 3 Jan 2021 23:29:26 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] No ghci return for homemade types
Message-ID: <20210103222926.GA9403@extensa>
Content-Type: text/plain; charset=utf-8
Il 03 gennaio 2021 alle 16:22 Lawrence Bottorff ha scritto:
> I've tried this
>
> data MyMaybe a = Just a | Nothing
>
> maybeHead :: [t] -> MyMaybe t
> maybeHead [] = Nothing
> maybeHead (x:_) = Just x
>
> and it evaluates fine. But this
>
> maybeHead [1,2,3]
>
> produces error:
> : • No instance for (Show (MyMaybe Integer))
> : arising from a use of ‘show’
> : • In the expression: show (maybeHead [1, 2, 3])
> : In an equation for ‘it’: it = show (maybeHead [1, 2, 3])
data MyMaybe a = Jus a | Not
deriving (Show, Eq)
Your text should have introduced to/warned you about deriving and
typeclasses.
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