poor compile time and memory usage with OverloadedString Text concatenation

(sorry, the template list seems broken here)

The following code does exhibits abysmally poor build performance and memory usage.

{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE OverloadedStrings #-}

module DocDebug where

import           Data.Text
import Data.String

foo :: Text
foo = "foo" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar" <> "bar"

Using ghc -fforce-recomp DocDebug.hs +RTS -s, the report returns 17s and ~2700 MiB of max resident memory.

Note that -O2 is important here, that's why I put it in the file itself.

Different observations:

• If I change the type signature from Text to String, the performances changes back to normal, i.e. 0.11s and 22 MiB. However we are building a String instead of Text.
• If I keep the Text type, but wraps everything in a fromString (i.e. foo = fromString $ "foo" <> "bar" <> "....), 0.11s and 22 MiB too. Same semantic, but 150 times faster and 120 times less memory used.

With -O1, results are similar, but not as dramatic:

• Initial code snipet, with -O1: 1s for 66 MiB
• With :: String, 0.14s, 23 MiB
• With fromString, 0.15s, 23 MiB.