Illegal unboxed string literal in pattern [GHC-69925]

When pattern matching, unboxed string literals (with the MagicHash postfix) are not allowed. Unboxed string literals are essentially C strings allocated outside of the Haskell heap, and they evaluate to pointers to the resulting strings. Thus,"foo"# is of type Addr#. While eqAddr# can be used to compare these pointers, two identically-written unboxed string literals typically do not point to the same address.

Examples

IllegalUnboxedString.hs:6:5: error: [GHC-69925]
    Illegal unboxed string literal in pattern:
    "a"#
  |
6 |     "a"# -> True
  |     ^^^^

IllegalUnboxedString.hs

{-# LANGUAGE MagicHash #-}

module IllegalUnboxedString where

import GHC.Prim

g :: Addr# -> Bool
g y = case y of
    "a"# -> True
    _    -> False

module IllegalUnboxedString where

g :: String -> Bool
g y = case y of
    "a" -> True
    _ -> False