6.4.8. Generalised Algebraic Data Types (GADTs)¶

GADTs
¶ Implies: MonoLocalBinds
,GADTSyntax
Since: 6.8.1 Allow use of Generalised Algebraic Data Types (GADTs).
Generalised Algebraic Data Types generalise ordinary algebraic data types by allowing constructors to have richer return types. Here is an example:
data Term a where
Lit :: Int > Term Int
Succ :: Term Int > Term Int
IsZero :: Term Int > Term Bool
If :: Term Bool > Term a > Term a > Term a
Pair :: Term a > Term b > Term (a,b)
Notice that the return type of the constructors is not always
Term a
, as is the case with ordinary data types. This generality
allows us to write a welltyped eval
function for these Terms
:
eval :: Term a > a
eval (Lit i) = i
eval (Succ t) = 1 + eval t
eval (IsZero t) = eval t == 0
eval (If b e1 e2) = if eval b then eval e1 else eval e2
eval (Pair e1 e2) = (eval e1, eval e2)
The key point about GADTs is that pattern matching causes type refinement. For example, in the right hand side of the equation
eval :: Term a > a
eval (Lit i) = ...
the type a
is refined to Int
. That’s the whole point! A precise
specification of the type rules is beyond what this user manual aspires
to, but the design closely follows that described in the paper Simple
unificationbased type inference for GADTs
(ICFP 2006). The general principle is this: type refinement is only carried
out based on usersupplied type annotations. So if no type signature is
supplied for eval
, no type refinement happens, and lots of obscure
error messages will occur. However, the refinement is quite general. For
example, if we had:
eval :: Term a > a > a
eval (Lit i) j = i+j
the pattern match causes the type a
to be refined to Int
(because of the type of the constructor Lit
), and that refinement
also applies to the type of j
, and the result type of the case
expression. Hence the addition i+j
is legal.
These and many other examples are given in papers by Hongwei Xi, and Tim Sheard. There is a longer introduction on the wiki, and Ralf Hinze’s Fun with phantom types also has a number of examples. Note that papers may use different notation to that implemented in GHC.
The rest of this section outlines the extensions to GHC that support
GADTs. The extension is enabled with GADTs
. The GADTs
extension
also sets GADTSyntax
and MonoLocalBinds
.
A GADT can only be declared using GADTstyle syntax (Declaring data types with explicit constructor signatures); the old Haskell 98 syntax for data declarations always declares an ordinary data type. The result type of each constructor must begin with the type constructor being defined, but for a GADT the arguments to the type constructor can be arbitrary monotypes. For example, in the
Term
data type above, the type of each constructor must end withTerm ty
, but thety
need not be a type variable (e.g. theLit
constructor).GADT constructors can include contexts and existential variables, generalising existential quantification (Existentially quantified data constructors). For example:
data SomeShow where SomeShow :: Show a => a > SomeShow  `a` is existential, as it does not appear in the return type data G a where MkG :: (a ~ Int) => a > a > G a  essentially the same as:  MkG :: Int > Int > G Int
It is permitted to declare an ordinary algebraic data type using GADTstyle syntax. What makes a GADT into a GADT is not the syntax, but rather the presence of data constructors whose result type is not just
T a b
, or which include contexts.A newtype may use GADTstyle syntax, but it must declare an ordinary data type, not a GADT. That is, the constructor must not bind existential variables (as per Existentially quantified data constructors) nor include a context.
You cannot use a
deriving
clause for a GADT; only for an ordinary data type (possibly using GADTstyle syntax). However, you can still use a Standalone deriving declarations declaration.As mentioned in Declaring data types with explicit constructor signatures, record syntax is supported. For example:
data Term a where Lit :: { val :: Int } > Term Int Succ :: { num :: Term Int } > Term Int Pred :: { num :: Term Int } > Term Int IsZero :: { arg :: Term Int } > Term Bool Pair :: { arg1 :: Term a , arg2 :: Term b } > Term (a,b) If :: { cnd :: Term Bool , tru :: Term a , fls :: Term a } > Term a
However, for GADTs there is the following additional constraint: every constructor that has a field
f
must have the same result type (modulo alpha conversion) Hence, in the above example, we cannot merge thenum
andarg
fields above into a single name. Although their field types are bothTerm Int
, their selector functions actually have different types:num :: Term Int > Term Int arg :: Term Bool > Term Int
See Field selectors and TypeApplications for a full description of how the types of toplevel field selectors are determined.
When patternmatching against data constructors drawn from a GADT, for example in a
case
expression, the following rules apply: The type of the scrutinee must be rigid.
 The type of the entire
case
expression must be rigid.  The type of any free variable mentioned in any of the
case
alternatives must be rigid.
A type is “rigid” if it is completely known to the compiler at its binding site. The easiest way to ensure that a variable has a rigid type is to give it a type signature. For more precise details see Simple unificationbased type inference for GADTs. The criteria implemented by GHC are given in the Appendix.
When GHC typechecks multiple patterns in a function clause, it typechecks each pattern in order from left to right. This has consequences for patterns that match on GADTs, such as in this example:
data U a where MkU :: U () v1 :: U a > a > a v1 MkU () = () v2 :: a > U a > a v2 () MkU = ()
Although
v1
andv2
may appear to be the same function but with differently ordered arguments, GHC will only typecheckv1
. This is because inv1
, GHC will first typecheck theMkU
pattern, which causesa
to be refined to()
. This refinement is what allows the subsequent()
pattern to typecheck at typea
. Inv2
, however, GHC first tries to typecheck the()
pattern, and becausea
has not been refined to()
yet, GHC concludes that()
is not of typea
.v2
can be made to typecheck by matching onMkU
before()
, like so:v2 :: a > U a > a v2 x MkU = case x of () > ()
Not only does GHC typecheck patterns from left to right, it also typechecks them from the outside in. This can be seen in this example:
data F x y where MkF :: y > F (Maybe z) y g :: F a a > a g (MkF Nothing) = Nothing
In the function clause for
g
, GHC first checksMkF
, the outermost pattern, followed by the innerNothing
pattern. This outsidein order can interact somewhat counterintuitively with Pattern type signatures. Consider the following variation ofg
:g2 :: F a a > a g2 (MkF Nothing :: F (Maybe z) (Maybe z)) = Nothing @z
The
g2
function attempts to use the pattern type signatureF (Maybe z) (Maybe z)
to bring the type variablez
into scope so that it can be used on the righthand side of the definition with Visible type application. However, GHC will reject the pattern type signature ing2
:• Couldn't match type ‘a’ with ‘Maybe z’ Expected: F a a Actual: F (Maybe z) (Maybe z)
Again, this is because of the outsidein order GHC uses when typechecking patterns. GHC first tries to check the pattern type signature
F (Maybe z) (Maybe z)
, but at that point, GHC has not refineda
to beMaybe z
, so GHC is unable to conclude thatF a a
is equal toF (Maybe z) (Maybe z)
. Here, theMkF
pattern is considered to be inside of the pattern type signature, so GHC cannot use the type refinement from theMkF
pattern when typechecking the pattern type signature.There are two possible ways to repair
g2
. One way is to use acase
expression to write a pattern signature after matching onMkF
, like so:g3 :: F a a > a g3 f@(MkF Nothing) = case f of (_ :: F (Maybe z) (Maybe z)) > Nothing @z
Another way is to use Type Applications in Patterns instead of a pattern type signature:
g4 :: F a a > a g4 (MkF @(Maybe z) Nothing) = Nothing @z
Here, the visible type argument
@(Maybe z)
indicates that they
in the type ofMkF :: y > F (Maybe z) y
should be instantiated toMaybe z
. In addition,@(Maybe z)
also bringsz
into scope. Althoughg4
no longer uses a pattern type signature, it accomplishes the same end result, as the righthand sideNothing @z
will typecheck successfully.