The Haskell 98 Report: Derived Instances The Haskell 98 Report
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D  Specification of Derived Instances

A derived instance is an instance declaration that is generated automatically in conjunction with a data or newtype declaration. The body of a derived instance declaration is derived syntactically from the definition of the associated type. Derived instances are possible only for classes known to the compiler: those defined in either the Prelude or a standard library. In this appendix, we describe the derivation of classes defined by the Prelude.

If T is an algebraic datatype declared by:

data cx => T u1 ... uk=K1 t11 ... t1k1 | ...| Kn tn1 ... tnkn
deriving (C1, ..., Cm)

(where m>=0 and the parentheses may be omitted if m=1) then a derived instance declaration is possible for a class C if these conditions hold:

  1. C is one of Eq, Ord, Enum, Bounded, Show, or Read.

  2. There is a context cx' such that cx' =>C tij holds for each of the constituent types tij.

  3. If C is Bounded, the type must be either an enumeration (all constructors must by nullary) or have only one constructor.

  4. If C is Enum, the type must be an enumeration.

  5. There must be no explicit instance declaration elsewhere in the program that makes T u1 ... uk an instance of C.
For the purposes of derived instances, a newtype declaration is treated as a data declaration with a single constructor.

If the deriving form is present, an instance declaration is automatically generated for T u1 ... uk over each class Ci. If the derived instance declaration is impossible for any of the Ci then a static error results. If no derived instances are required, the deriving form may be omitted or the form deriving () may be used.

Each derived instance declaration will have the form:

instance (cx, C'1 u'1, ..., C'j u'j ) => Ci (T u1 ... uk) where { d }

where d is derived automatically depending on Ci and the data type declaration for T (as will be described in the remainder of this section), and u'1 through u'j form a subset of u1 through uk. When inferring the context for the derived instances, type synonyms must be expanded out first. Free names in the declarations d are all defined in the Prelude; the qualifier `Prelude.' is implicit here. The remaining details of the derived instances for each of the derivable Prelude classes are now given.

D.1  Derived instances of Eq and Ord.

The class methods automatically introduced by derived instances of Eq and Ord are (==), (/=), compare, (<), (<=), (>), (>=), max, and min. The latter seven operators are defined so as to compare their arguments lexicographically with respect to the constructor set given, with earlier constructors in the datatype declaration counting as smaller than later ones. For example, for the Bool datatype, we have that (True > False) == True.

Derived comparisons always traverse constructors from left to right. These examples illustrate this property:

(1,undefined) == (2,undefined) 
=>    False
(undefined,1) == (undefined,2) 
=>    _|_

D.2  Derived instances of Enum

Derived instance declarations for the class Enum are only possible for enumerations. Enum introduces the class methods succ, pred, toEnum, fromEnum, enumFrom, enumFromThen, enumFromTo, and enumFromThenTo. The latter four are used to define arithmetic sequences as described in Section 3.10.

The nullary constructors are assumed to be numbered left-to-right with the indices 0 through n-1. The succ and pred operators give the successor and predecessor respectively of a value, under this numbering scheme. It is an error to apply succ to the maximum element, or pred to the minimum element.

The toEnum and fromEnum operators map enumerated values to and from the Int type.

enumFrom n returns a list corresponding to the complete enumeration of n's type starting at the value n. Similarly, enumFromThen n n' is the enumeration starting at n, but with second element n', and with subsequent elements generated at a spacing equal to the difference between n and n'. enumFromTo and enumFromThenTo are as defined by the default class methods for Enum (see Figure 5, page ). For example, given the datatype:

data  Color = Red | Orange | Yellow | Green  deriving (Enum)

we would have:

[Orange ..]         ==  [Orange, Yellow, Green]
fromEnum Yellow     ==  2

D.3  Derived instances of Bounded.

The Bounded class introduces the class methods minBound and maxBound, which define the minimal and maximal elements of the type. For an enumeration, the first and last constructors listed in the data declaration are the bounds. For a type with a single constructor, the constructor is applied to the bounds for the constituent types. For example, the following datatype:

data  Pair a b = Pair a b deriving Bounded

would generate the following Bounded instance:

instance (Bounded a,Bounded b) => Bounded (Pair a b) where
  minBound = Pair minBound minBound
  maxBound = Pair maxBound maxBound

D.4  Derived instances of Read and Show.

The class methods automatically introduced by derived instances of Read and Show are showsPrec, readsPrec, showList, and readList. They are used to coerce values into strings and parse strings into values.

The function showsPrec d x r accepts a precedence level d (a number from 0 to 10), a value x, and a string r. It returns a string representing x concatenated to r. showsPrec satisfies the law:

showsPrec d x r ++ s  ==  showsPrec d x (r ++ s)

The representation will be enclosed in parentheses if the precedence of the top-level constructor operator in x is less than d. Thus, if d is 0 then the result is never surrounded in parentheses; if d is 10 it is always surrounded in parentheses, unless it is an atomic expression. The extra parameter r is essential if tree-like structures are to be printed in linear time rather than time quadratic in the size of the tree.

The function readsPrec d s accepts a precedence level d (a number from 0 to 10) and a string s, and attempts to parse a value from the front of the string, returning a list of (parsed value, remaining string) pairs. If there is no successful parse, the returned list is empty. It should be the case that

  fst (head (readsPrec d (showsPrec d x r))) == x

That is, readsPrec should be able to parse the string produced by showsPrec, and should deliver the value that showsPrec started with.

showList and readList allow lists of objects to be represented using non-standard denotations. This is especially useful for strings (lists of Char).

readsPrec will parse any valid representation of the standard types apart from lists, for which only the bracketed form [...] is accepted. See Appendix A for full details.

A precise definition of the derived Read and Show instances for general types is beyond the scope of this report. However, the derived Read and Show instances have the following properties:

The derived Read and Show instances may be unsuitable for some uses. Some problems include:

D.5  An Example

As a complete example, consider a tree datatype:

data Tree a = Leaf a | Tree a :^: Tree a
     deriving (Eq, Ord, Read, Show)

Automatic derivation of instance declarations for Bounded and Enum are not possible, as Tree is not an enumeration or single-constructor datatype. The complete instance declarations for Tree are shown in Figure 8, Note the implicit use of default class method definitions---for example, only <= is defined for Ord, with the other class methods (<, >, >=, max, and min) being defined by the defaults given in the class declaration shown in Figure 5 (page ).

infix 4 :^:
data Tree a =  Leaf a  |  Tree a :^: Tree a

instance (Eq a) => Eq (Tree a) where
Leaf m == Leaf n  =  m==n
u:^:v  == x:^:y   =  u==x && v==y
             _ == _       =  False

instance (Ord a) => Ord (Tree a) where
Leaf m <= Leaf n  =  m<=n
Leaf m <= x:^:y   =  True
u:^:v  <= Leaf n  =  False
u:^:v  <= x:^:y   =  u<x || u==x && v<=y

instance (Show a) => Show (Tree a) where

        showsPrec d (Leaf m) = showParen (d >= 10) showStr
             showStr = showString "Leaf " . showsPrec 10 m

        showsPrec d (u :^: v) = showParen (d > 4) showStr
             showStr = showsPrec 5 u . 
                       showString " :^: " .
                       showsPrec 5 v

instance (Read a) => Read (Tree a) where

readsPrec d r =  readParen (d > 4)
 (\r -> [(u:^:v,w) |
 (u,s) <- readsPrec 5 r,
 (":^:",t) <- lex s,
 (v,w) <- readsPrec 5 t]) r

      ++ readParen (d > 9)
 (\r -> [(Leaf m,t) |
 ("Leaf",s) <- lex r,
 (m,t) <- readsPrec 10 s]) r

Figure 8

Example of Derived Instances

The Haskell 98 Report
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1 February, 1999