Control.Lens.TH

Constructing Lenses Automatically

makeLenses :: Name -> Q [Dec]

Build lenses (and traversals) with a sensible default configuration.

e.g.

data FooBar
  = Foo { _x, _y :: Int }
  | Bar { _x :: Int }
makeLenses ''FooBar

will create

x :: Lens' FooBar Int
x f (Foo a b) = (a' -> Foo a' b) <$> f a
x f (Bar a)   = Bar <$> f a
y :: Traversal' FooBar Int
y f (Foo a b) = (b' -> Foo a  b') <$> f b
y _ c@(Bar _) = pure c

makeLenses = makeLensesWith lensRules

makeLensesFor :: [(String, String)] -> Name -> Q [Dec]

Derive lenses and traversals, specifying explicit pairings of (fieldName, lensName).

If you map multiple names to the same label, and it is present in the same constructor then this will generate a Traversal.

e.g.

makeLensesFor [("_foo", "fooLens"), ("baz", "lbaz")] ''Foo
makeLensesFor [("_barX", "bar"), ("_barY", "bar")] ''Bar

makeClassy :: Name -> Q [Dec]

Make lenses and traversals for a type, and create a class when the type has no arguments.

e.g.

data Foo = Foo { _fooX, _fooY :: Int }
makeClassy ''Foo

will create

class HasFoo t where
  foo :: Simple Lens t Foo
instance HasFoo Foo where foo = id
fooX, fooY :: HasFoo t => Simple Lens t Int

makeClassy = makeLensesWith classyRules

makeClassyFor :: String -> String -> [(String, String)] -> Name -> Q [Dec]

Derive lenses and traversals, using a named wrapper class, and specifying explicit pairings of (fieldName, traversalName).

Example usage:

makeClassyFor "HasFoo" "foo" [("_foo", "fooLens"), ("bar", "lbar")] ''Foo

makeClassy_ :: Name -> Q [Dec]

Make lenses and traversals for a type, and create a class when the type has no arguments. Works the same as makeClassy except that (a) it expects that record field names do not begin with an underscore, (b) all record fields are made into lenses, and (c) the resulting lens is prefixed with an underscore.

makeIso :: Name -> Q [Dec]

Make a top level isomorphism injecting into the type.

The supplied name is required to be for a type with a single constructor that has a single argument.

e.g.

newtype List a = List [a]
makeIso ''List

will create

list :: Iso [a] [b] (List a) (List b)

makeIso = makeLensesWith isoRules

makePrisms :: Name -> Q [Dec]

Generate a Prism for each constructor of a data type.

e.g.

data FooBarBaz a
  = Foo Int
  | Bar a
  | Baz Int Char
makePrisms ''FooBarBaz

will create

_Foo :: Prism' (FooBarBaz a) Int
_Bar :: Prism (FooBarBaz a) (FooBarBaz b) a b
_Baz :: Prism' (FooBarBaz a) (Int, Char)

makeWrapped :: Name -> DecsQ

Build Wrapped instance for a given newtype

makeFields :: Name -> Q [Dec]

Generate overloaded field accessors.

e.g

data Foo a = Foo { _fooX :: Int, _fooY : a }
newtype Bar = Bar { _barX :: Char }
makeFields ''Foo
makeFields ''Bar

will create

_fooXLens :: Lens' (Foo a) Int
_fooYLens :: Lens (Foo a) (Foo b) a b
class HasX s a | s -> a where
  x :: Lens' s a
instance HasX (Foo a) Int where
  x = _fooXLens
class HasY s a | s -> a where
  y :: Lens' s a
instance HasY (Foo a) a where
  y = _fooYLens
_barXLens :: Iso' Bar Char
instance HasX Bar Char where
  x = _barXLens

makeFields = makeFieldsWith defaultFieldRules

Constructing Lenses Given a Declaration Quote

declareLenses :: Q [Dec] -> Q [Dec]

Make lenses for all records in the given declaration quote. All record syntax in the input will be stripped off.

e.g.

declareLenses [d|
  data Foo = Foo { fooX, fooY :: Int }
    deriving Show
  |]

will create

data Foo = Foo Int Int deriving Show
fooX, fooY :: Lens' Foo Int

 declareLenses = declareLensesWith (lensRules & lensField .~ Just)

declareLensesFor :: [(String, String)] -> Q [Dec] -> Q [Dec]

Similar to makeLensesFor, but takes a declaration quote.

declareClassy :: Q [Dec] -> Q [Dec]

For each record in the declaration quote, make lenses and traversals for it, and create a class when the type has no arguments. All record syntax in the input will be stripped off.

e.g.

declareClassy [d|
  data Foo = Foo { fooX, fooY :: Int }
    deriving Show
  |]

will create

data Foo = Foo Int Int deriving Show
class HasFoo t where
  foo :: Lens' t Foo
instance HasFoo Foo where foo = id
fooX, fooY :: HasFoo t => Lens' t Int

 declareClassy = declareLensesWith (classyRules & lensField .~ Just)

declareClassyFor :: [(String, (String, String))] -> [(String, String)] -> Q [Dec] -> Q [Dec]

Similar to makeClassyFor, but takes a declaration quote.

declareIso :: Q [Dec] -> Q [Dec]

For each datatype declaration, make a top level isomorphism injecting into the type. The types are required to be for a type with a single constructor that has a single argument.

All record syntax in the input will be stripped off.

e.g.

declareIso [d|
  newtype WrappedInt = Wrap { unrwap :: Int }
  newtype List a = List [a]
  |]

will create

newtype WrappedList = Wrap Int
newtype List a = List [a]
wrap :: Iso' Int WrappedInt
unwrap :: Iso' WrappedInt Int
list :: Iso [a] [b] (List a) (List b)

 declareIso = declareLensesWith (isoRules & lensField .~ Just)

declarePrisms :: Q [Dec] -> Q [Dec]

Generate a Prism for each constructor of each data type.

e.g.

declarePrisms [d|
  data Exp = Lit Int | Var String | Lambda{ bound::String, body::Exp }
  |]

will create

data Exp = Lit Int | Var String | Lambda { bound::String, body::Exp }
_Lit :: Prism' Exp Int
_Var :: Prism' Exp String
_Lambda :: Prism' Exp (String, Exp)

declareWrapped :: Q [Dec] -> Q [Dec]

Build Wrapped instance for each newtype.

declareFields :: Q [Dec] -> Q [Dec]

 declareFields = declareFieldsWith defaultFieldRules

Configuring Lenses

makeLensesWith :: LensRules -> Name -> Q [Dec]

Build lenses with a custom configuration.

makeFieldsWith :: FieldRules -> Name -> Q [Dec]

Make fields with the specified FieldRules.

declareLensesWith :: LensRules -> Q [Dec] -> Q [Dec]

Declare lenses for each records in the given declarations, using the specified LensRules. Any record syntax in the input will be stripped off.

declareFieldsWith :: FieldRules -> Q [Dec] -> Q [Dec]

Declare fields for each records in the given declarations, using the specified FieldRules. Any record syntax in the input will be stripped off.

defaultRules :: LensRules

Default LensRules.

defaultFieldRules :: FieldRules

 defaultFieldRules = camelCaseFields

camelCaseFields :: FieldRules

Field rules for fields in the form prefixFieldname or _prefixFieldname If you want all fields to be lensed, then there is no reason to use an _ before the prefix. If any of the record fields leads with an _ then it is assume a field without an _ should not have a lens created.

underscoreFields :: FieldRules

Field rules for fields in the form _prefix_fieldname

data LensRules

This configuration describes the options we'll be using to make isomorphisms or lenses.

Constructors

LensRules (String -> Maybe String) (String -> Maybe String) (String -> Maybe (String, String)) (Set LensFlag)

data FieldRules

Constructors

FieldRules ([String] -> String -> Maybe String) (String -> String) (String -> Maybe String) (String -> Maybe String)

lensRules :: LensRules

Rules for making fairly simple partial lenses, ignoring the special cases for isomorphisms and traversals, and not making any classes.

classyRules :: LensRules

Rules for making lenses and traversals that precompose another Lens.

classyRules_ :: LensRules

isoRules :: LensRules

Rules for making an isomorphism from a data type.

lensIso :: Lens' LensRules (String -> Maybe String)

Lens' to access the convention for naming top level isomorphisms in our LensRules.

Defaults to lowercasing the first letter of the constructor.

lensField :: Lens' LensRules (String -> Maybe String)

Lens' to access the convention for naming fields in our LensRules.

Defaults to stripping the _ off of the field name, lowercasing the name, and rejecting the field if it doesn't start with an '_'.

lensClass :: Lens' LensRules (String -> Maybe (String, String))

Retrieve options such as the name of the class and method to put in it to build a class around monomorphic data types.

lensFlags :: Lens' LensRules (Set LensFlag)

Retrieve options such as the name of the class and method to put in it to build a class around monomorphic data types.

data LensFlag

Flags for Lens construction

Constructors

SimpleLenses

PartialLenses

BuildTraversals

SingletonAndField

SingletonIso

BackwardIso

HandleSingletons

SingletonRequired

CreateClass

CreateInstance

ClassRequired

GenerateSignatures

Instances

Eq LensFlag

Ord LensFlag

Read LensFlag

Show LensFlag

simpleLenses :: Lens' LensRules Bool

Only Generate valid Simple lenses.

partialLenses :: Lens' LensRules Bool

Enables the generation of partial lenses, generating runtime errors for every constructor that does not have a valid definition for the Lens. This occurs when the constructor lacks the field, or has multiple fields mapped to the same Lens.

buildTraversals :: Lens' LensRules Bool

In the situations that a Lens would be partial, when partialLenses is used, this flag instead causes traversals to be generated. Only one can be used, and if neither are, then compile-time errors are generated.

handleSingletons :: Lens' LensRules Bool

Handle singleton constructors specially.

singletonIso :: Lens' LensRules Bool

Use Iso for singleton constructors.

backwardIso :: Lens' LensRules Bool

When generating an Iso put the field type as the "outer" type.

singletonRequired :: Lens' LensRules Bool

Expect a single constructor, single field newtype or data type.

createClass :: Lens' LensRules Bool

Create the class if the constructor is Simple and the lensClass rule matches.

createInstance :: Lens' LensRules Bool

Create the instance if the constructor is Simple and the lensClass rule matches.

classRequired :: Lens' LensRules Bool

Die if the lensClass fails to match.

singletonAndField :: Lens' LensRules Bool

When building a singleton Iso (or Lens) for a record constructor, build both the Iso (or Lens) for the record and the one for the field.

generateSignatures :: Lens' LensRules Bool

Indicate whether or not to supply the signatures for the generated lenses.

Disabling this can be useful if you want to provide a more restricted type signature or if you want to supply hand-written haddocks.