Question

Given a datatype

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

An incomplete expression passes typechecking — e.g.

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

main = print foo

Typechecks (emitting a warning about the incomplete record), then (obviously) crashes when the expression is encountered. Why does it pass? Without record syntax it doesn’t (i.e. bar = Foo 5 :: Foo).

Answer 1

The Haskell 2010 report says in section 3.15.2 Construction Using Field Labels

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.

So it’s part of the language specification and a compiler must accept the code. All fields are initialized, just some are initialised with undefined.

foo = Foo{ one = 5 }

is equivalent to

foo = Foo 5 undefined

A nice compiler will warn you about it if you ask it to. If you want an error, make the fields strict.