“=~” raise “No instance for (RegexContext Regex [Char] [String])”

by 
ghci> getAllTextMatches ("foo" =~ "o" :: AllTextMatches [] String)
["o", "o"]

I haven’t used regexes in Haskell much (which is I think Dan Burton’s answer is more idiomatic).

So the way I figured this out is I looked at your type error No instance for (RegexContext Regex [Char] [String]), and popped into ghci:

ghci> :t (=~)
(=~)
  :: (RegexMaker Regex CompOption ExecOption source,
      RegexContext Regex source1 target) =>
     source1 -> source -> target

So RegexContext Regex [Char] [String] is a class that includes the return type of "foo" =~ "o" :: [String]. So I looked to see what instances of this class did exist, so I could find out what the return value was allowed to be:

ghci> :i RegexContext
class RegexLike
        regex source => RegexContext regex source target where
  match :: regex -> source -> target
  matchM :: Monad m => regex -> source -> m target
        -- Defined in Text.Regex.Base.RegexLike
instance RegexContext Regex String String
  -- Defined in Text.Regex.Posix.String
instance RegexLike a b => RegexContext a b [[b]]
  -- Defined in Text.Regex.Base.Context
...
instance RegexLike a b => RegexContext a b (AllTextMatches [] b)
  -- Defined in Text.Regex.Base.Context
...

The AllTextMatches name seemed to indicate what you were looking for, so I checked that out:

ghci> :i AllTextMatches
newtype AllTextMatches f b
  = AllTextMatches {getAllTextMatches :: f b}
        -- Defined in Text.Regex.Base.RegexLike
instance RegexLike a b => RegexContext a b (AllTextMatches [] b)
  -- Defined in Text.Regex.Base.Context

So this was the type to use to extract all the text matches, as I suspected. All I needed to do was indicate that I wanted a return value of that type.

Note also the possible return type of [[b]], which I assume returns a list of lists containing each complete match and all its submatches:

ghci> "foo" =~ "o" :: [[String]]
[["o"],["o"]]
ghci> "foo bar baz" =~ "[aeiou](.)" :: [[String]]
[["oo","o"],["ar","r"],["az","z"]]

So maybe that’s the type you meant to use, instead of [String]. I could see [String] as being slightly ambiguous when [[String]] existed – should "foo bar baz" =~ "[aeiou](.)" :: [String] be fst ("foo bar baz" =~ "[aeiou](.)" :: [[String]]) or map fst ("foo bar baz" =~ "[aeiou](.)" :: [[String]]).

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