Top-level strict or unlifted binds not allowed [GHC-48099]

In Haskell, top-level bindings must be represented by thunks because the evaluation time of things that can fail is hard to understand when they’re part of a module. This rules out strict bindings as well as unlifted bindings.

Examples

Top-level strict bindings

Message

Strict.hs:3:1: error: [GHC-48099]
    Top-level strict bindings aren't allowed: !x = 5
  |
3 | !x = 5
  | ^^^^^^

Explanation

The top-level binding of x is a strict pattern, which is not allowed. This can be fixed by not having a strict pattern for a top-level binding.

Strict.hs

Before

module Strict where

!x = 5

After

module Strict where

x = 5

Unlifted top-level binding

Message

Unlifted.hs:4:1: error: [GHC-48099]
    Top-level bindings for unlifted types aren't allowed: x = 4#
  |
4 | x = 4#
  | ^^^^^^

Explanation

Top-level bindings are not allowed to be unlifted types, because they always need to be a thunk. To fix the issue, use a lifted type.

Unlifted.hs

Before

{-# LANGUAGE MagicHash #-}
module Unlifted where

x = 4#

After


module Unlifted where

x = 4