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POHow to "show" a constructor in lower case
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copied!<p>Suppose I'd want to create a new data type and make the constructors showable, only in lower case instead of their upper case definition. For example:</p> <pre><code>data Day = Monday | Tuesday | Wednesday| Thursday | Friday | Saturday | Sunday </code></pre> <p>By adding <em>deriving Show</em>, ghci would print them as "Monday, Tuesday.. etc." To get it to show "monday, tuesday.. etc" I've tried to make a special instance of show:</p> <pre><code>import Data.Char strToLower :: [Char] -> [Char] strToLower (x:xs) = toLower x : strToLower xs strToLower [] = [] instance Show Day where show d = strToLower (show d) </code></pre> <p>where the first occurrence of show should designate my new amended show function (which will be called everytime I print) whereas for the second I intend the normally derived version of show, to get from the constructor name to a String.</p> <p>Of course this doesn't work (circular definition) as the ghci has no clue to my separate meanings of the word "show" but I can't figure out how to let him know the distinction, for both versions need to be named show, the first because that's what print calls to and the second because it's a predefined haskell function which can give me a String out of a constructor name. I've tried </p> <pre><code>show d = strToLower ((showsPrec 0 d) "") </code></pre> <p>but this comes down to the same circular definition, at least that's what I guess from the ghci getting stuck in a loop.</p> <p>I understand why constructor names need to begin with an upper case letter, but showing them lower case shouldn't be a problem, should it? I know I could just define my show function for every case separately, e.g. <code>show Monday = "monday"</code> <code>show Tuesday = "tuesday"</code> etc, but I'm only using the days of the week as an example here and my real data type consists of 64 constructors so I think it would be more elegant to solve it differently somehow.</p> <p>Is it possible to dig into the haskell definition of show and alter a copy of that code? This is the only possible solution I can think of but I don't know how to do it, if it is possible at all. Probably not. So other solutions are very welcome as well!</p> <p>Thank you for taking your time,</p> <p>Jelle (Haskell beginner)</p>

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