haskell - 为什么要对该记录进行类型检查？

Question

给定数据类型

data Foo = Foo { one :: Int, two :: String } deriving (Show)

不完整的表达式通过类型检查——例如

foo :: Foo
foo = Foo { one = 5 }

main = print foo

类型检查(发出有关不完整记录的警告)，然后(显然)在遇到表达式时崩溃。为什么会通过？如果没有记录语法，则不会(即 bar = Foo 5::Foo)。

Answer 1

Haskell 2010 报告在 3.15.2 Construction Using Field Labels 部分中提到

A constructor with labeled fields may be used to construct a value in which the components are specified by name rather than by position. Unlike the braces used in declaration lists, these are not subject to layout; the { and } characters must be explicit. (This is also true of field updates and field patterns.) Construction using field labels is subject to the following constraints: [...]

• Fields not mentioned are initialized to ⊥.

• A compile-time error occurs when any strict fields (fields whose declared types are prefixed by !) are omitted during construction.

因此它是语言规范的一部分，编译器必须接受该代码。所有字段均已初始化，仅部分字段使用 undefined 进行初始化。

foo = Foo{ one = 5 }

相当于

foo = Foo 5 undefined

如果你要求的话，一个好的编译器会警告你。如果您想要错误，请严格指定字段。