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I have different data types that I have defined, and I want to make them all an instance of a class. is it possible to make a list of types and map instance over them instead of having to declare them all individually?
I mean something like this:
data Type1 = ...
data Type2 = ...
map (instance ClassName) [Type1, Type2]
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One is of type (String,Int) , whereas the other is (Int,String) . This has implications for building up lists of tuples. We could very well have lists like [("a",1),("b",9),("c",9)] , but Haskell cannot have a list like [("a",1),(2,"b"),(9,"c")] . Which of these are valid Haskell, and why?
If show is ok to print your elements, you can use putStr ( unlines $ map show [(1,"A"),(2,"B"),(3,"C"),(4,"D")]) but you can replace show by any funtion that'll take your custom type and return a string.
In Haskell, lists are a homogenous data structure. It stores several elements of the same type. That means that we can have a list of integers or a list of characters but we can't have a list that has a few integers and then a few characters.
Well actually you can, with something like
{-# LANGUAGE TemplateHaskell #-}
module T where
class C a
data X = X
data Y = Y
data Z = Z
$(fmap concat $ mapM (\t -> [d|instance C $t|]) [[t|X|], [t|Y|], [t|Z|]])
but it strikes me as massive overkill unless you actually need them to be generated automatically (e.g., the list of types might vary depending on something).
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