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I found myself an idiot for such a long time I was not aware of this use of asTypeOf function.

> module AsTypeOf where

When we have a type with multiple varialbles, e.g. Either:

> rightChar = Right 'a'

In ghci:

ghci> :t rightChar
rightChar :: Either a Char

We are yet not sure for remaining type variables. In above case, we still do not have a clue to fix the type of Left constructor.

One way to fix the type used for Left is wrapping multiple values in single list:

> rightCharInList = [rightChar, Left True]

Showing the type:

ghci> :t rightCharInList
rightCharInList :: [Either Bool Char]

Or another, using asTypeOf instead of list:

> rightChar' = rightChar `asTypeOf` Left True

Now the type for Left constructor is fixed, value is identical to rightChar:

ghci> rightChar'
Right 'a'
ghci> :t rightChar'
rightChar' :: Either Bool Char