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Im new to learning Haskell and have been using the book "Haskell-The Craft of Functional Programming 3rd edition" and am stuck on Chapter 12 regarding regular expressions, problem 12.16 in particular:

implement the following functions: option,plus:: RegExp->RegExp -where option e matches 0 or 1 occurence of e and plus e matches 1 or more occurence of e.

Im not sure where to evenbegin so any help would be appreciated! Thanks

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  • Check this resource Commented Feb 22, 2016 at 22:59
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    I don't know that this will help with the particular exercise, but regex-applicative offers a particularly pleasant API for dealing with regular languages. Commented Feb 22, 2016 at 23:38

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According to the star function, plus function should be like this:

plus :: RegExp -> RegExp plus e = e <*> star e

Where a <*> b means "a followed by b", and star matches 0 or multiple occurences.

Try to use epsilon and (|||) for option function.

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