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Effective Haskell: no `Show expr` arising from print

molleweide

molleweide

Effective Haskell: no `Show expr` arising from print

Hi @RebeccaSkinner,

In for example the Calculator segment of chapter 4, if I run the examples in GHCI this error is returned:

    λ: Add (Lit 5) (Lit 4)

    <interactive>:159:1: error:
        • No instance for (Show Expr) arising from a use of ‘print’
        • In a stmt of an interactive GHCi command: print it

I understand what it means but personally I am not sure how to make all examples go past this. It seems by your examples in the book also that you don’t have this issue?

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First Post!

RebeccaSkinner

RebeccaSkinner

Author of Effective Haskell

Thanks for catching this! I’ll take a look and push a fix in the next update!

In the mean time, you can work around this bug by adding deriving Show after your definition of Expr. For example:

data Expr = Lit Int
    | Sub Expr Expr
    | Add Expr Expr
    | Mul Expr Expr
    | Div Expr Expr
    deriving Show
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