CODEWITHSUNDEEP
haskell
user9645
user9645

Reputation: 6796

How to wrap a Text value with quotes in Haskell

I have some broken Haskell code and have never even heard of Haskell before now.

The code is generating a DB query string and is failing to properly quote the identifiers.

So I simply want to put quotes around the identifiers.

Here is the existing code:

(this part in a separate file):

data PgArg = PgArg {
  pgaName :: Text
, pgaType :: Text
, pgaReq  :: Bool
} deriving (Show, Eq, Ord)

(then the broken part in another file):

    unwords [
         "_args_record AS (",
         "SELECT * FROM " <> (if isObject then "json_to_record" else "json_to_recordset") <> "($1)",
         "AS _(" <> intercalate ", " ((\a -> pgaName a <> " " <> pgaType a) <$> pgArgs) <> ")",
     ")"]

I tried a few hacks, ultimately winding up with:

    unwords [
         "_args_record AS (",
         "SELECT * FROM " <> (if isObject then "json_to_record" else "json_to_recordset") <> "($1)",
         "AS _(" <> intercalate ", " ((\a -> (["\""] ++ (pgaName a) ++ ["\""]) <> " " <> pgaType a) <$> pgArgs) <> ")",
     ")"]

So basically I just changed:

pgaName a

To:

(["\""] ++ (pgaName a) ++ ["\""])

-- Anyway it fails with a type mismatch

error:
    * Couldn't match expected type `[Char]' with actual type `Text'
    * In the first argument of `(++)', namely `(pgaName a)'
      In the second argument of `(++)', namely `(pgaName a) ++ ["\""]'
      In the first argument of `(<>)', namely
        `(["\""] ++ (pgaName a) ++ ["\""])'

Any ideas how to fix this correctly?

Upvotes: 0

Views: 400

Answers (1)

arrowd
arrowd

Reputation: 34391

You faced the problem because (++) operates on lists ([]), and while Strings are just [Char], Text is not.

Fortunately, there is generic append operator coming from Monoid class called (<>). So, as @4castle said, just use "\"" <> pgaName a <> "\"" to concatenate Texts.

Upvotes: 3

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