Haskell Language Record Syntax Basic Syntax
Example
Records are an extension of sum algebraic data type that allow fields to be named:
data StandardType = StandardType String Int Bool --standard way to create a sum type
data RecordType = RecordType { -- the same sum type with record syntax
aString :: String
, aNumber :: Int
, isTrue :: Bool
}
The field names can then be used to get the named field out of the record
> let r = RecordType {aString = "Foobar", aNumber= 42, isTrue = True}
> :t r
r :: RecordType
> :t aString
aString :: RecordType -> String
> aString r
"Foobar"
Records can be pattern matched against
case r of
RecordType{aNumber = x, aString=str} -> ... -- x = 42, str = "Foobar"
Notice that not all fields need be named
Records are created by naming their fields, but can also be created as ordinary sum types (often useful when the number of fields is small and not likely to change)
r = RecordType {aString = "Foobar", aNumber= 42, isTrue = True}
r' = RecordType "Foobar" 42 True
If a record is created without a named field, the compiler will issue a warning, and the resulting value will be undefined.
> let r = RecordType {aString = "Foobar", aNumber= 42}
<interactive>:1:9: Warning:
Fields of RecordType not initialized: isTrue
> isTrue r
Error 'undefined'
A field of a record can be updated by setting its value. Unmentioned fields do not change.
> let r = RecordType {aString = "Foobar", aNumber= 42, isTrue = True}
> let r' = r{aNumber=117}
-- r'{aString = "Foobar", aNumber= 117, isTrue = True}
It is often useful to create lenses for complicated record types.
