Haskell Language Strictness Strict fields


In a data declaration, prefixing a type with a bang (!) makes the field a strict field. When the data constructor is applied, those fields will be evaluated to weak head normal form, so the data in the fields is guaranteed to always be in weak head normal form.

Strict fields can be used in both record and non-record types:

data User = User
    { identifier :: !Int
    , firstName :: !Text
    , lastName :: !Text

data T = MkT !Int !Int