I was writing some Haskell code where I was constructing a value with lots of record fields, but only cared about one of them initially.

data Type = Type { field1 :: Type1, field2 :: Type2 }

But, omitting the irrelevant fields from the initializer causes a big "Fields not initialised" warning.

So, I figured that, instead of writing out

Type { field1 = value, field2 = undefined }

which was verbose, or

Type { field1 = value }

which resulted in the big warning, I could use

undefined { field1 = value }

which did eliminate the warning.

However,

field1 (undefined { field1 = value })

resulted in an exception. I had expected it to be equivalent to

field1 (Type { field1 = value, field2 = field2 undefined })

but it's not. Since types can have multiple constructors, it's actually equivalent to

field1 (case undefined of { Type { field2 = field2 } -> Type { field1 = value, field2 = field2 })

according to section 3.16.3 in the Haskell report.