Haskell: use Data.Text.replace to replace only the first occurrence of a text value

Question

How do I only replace the first occurrence of a "substring" (actually Data.Text.Text) in a Text value in Haskell in the most efficient way?

Answer 1

You can breakOn the substring,

breakOn :: Text -> Text -> (Text, Text)

to split the Text into two parts at the first occurrence of the pattern, then replace the substring at the start of the second component:

replaceOne :: Text -> Text -> Text -> Text
replaceOne pattern substitution text
  | T.null back = text    -- pattern doesn't occur
  | otherwise = T.concat [front, substitution, T.drop (T.length pattern) back] 
    where
      (front, back) = breakOn pattern text