Question

I have a function like this:

example           :: [Char] -> [Char]
example myString  = ...................
                    where
                      pat        = "something"
                      returnList = myString =~ pat :: [(MatchOffset,MatchLength)]

My problem is that I don’t know how to store the values I get back from calling myString =~ pat :: [(MatchOffset,MatchLength)]

I can’t just store it in a single variable name as I have done here, but I’m not sure how I do store it.

It currently gives this error:

No instance for (RegexContext
                   Regex [Char] [(MatchOffset, MatchLength)])
  arising from a use of `=~'
Possible fix:
  add an instance declaration for
  (RegexContext Regex [Char] [(MatchOffset, MatchLength)])
In the expression: myString =~ pat :: [(MatchOffset, MatchLength)]
In an equation for `returnList':
    returnList = myString =~ pat :: [(MatchOffset, MatchLength)]
In an equation for `example':
    example myString
      = ....................
      where
          pat = "something"
          returnList = myString =~ pat :: [(MatchOffset, MatchLength)]

Answer 1

Looking at the instances available for class RegexLike, the value you want is probably of type AllMatches [] (MatchOffset, MatchLength), which simply wraps the list of tuples (MatchOffset, MatchLength) to a newtype. The list can then be accessed using the getAllMatches function. So you can do this:

returnList = getAllMatches (myString =~ pat) :: [(MatchOffset,MatchLength)]