Lens.Micro.TH

microlens-th-0.4.3.16: Automatic generation of record lenses for microlens

Copyright | (C) 2013-2016 Eric Mertens Edward Kmett Artyom Kazak; 2018 Monadfix
License | BSD-style (see the file LICENSE)
Safe Haskell | Trustworthy
Language | Haskell2010

Lens.Micro.TH

Contents

- Dealing with “not in scope” errors
- Using this module in GHCi
- `SimpleGetter` and `SimpleFold`
- Generating lenses
- Default lens rules
- Configuring lens rules

Description

Synopsis

- makeLenses :: Name -> DecsQ
- makeLensesFor :: [(String, String)] -> Name -> DecsQ
- makeLensesWith :: LensRules -> Name -> DecsQ
- makeFields :: Name -> DecsQ
- makeClassy :: Name -> DecsQ
- data LensRules
- data DefName
  - = TopName Name
  - | MethodName Name Name
- lensRules :: LensRules
- lensRulesFor :: [(String, String)] -> LensRules
- classyRules :: LensRules
- camelCaseFields :: LensRules
- abbreviatedFields :: LensRules
- lensField :: Lens' LensRules (Name -> [Name] -> Name -> [DefName])
- lensClass :: Lens' LensRules (Name -> Maybe (Name, Name))
- createClass :: Lens' LensRules Bool
- simpleLenses :: Lens' LensRules Bool
- generateSignatures :: Lens' LensRules Bool
- generateUpdateableOptics :: Lens' LensRules Bool
- generateLazyPatterns :: Lens' LensRules Bool

# Dealing with “not in scope” errors

When you use Template Haskell, the order of declarations suddenly starts to matter. For instance, if you try to use `makeLenses`, `makeFields`, etc before the type is defined, you'll get a “not in scope” error:

    makeLenses ''Foo

    data Foo = Foo {_foo :: Int}

    Not in scope: type constructor or class ‘Foo’ …
        In the Template Haskell quotation ''Foo

You can't refer to generated lenses before you call `makeLenses`, either:

    data Foo = Foo {_foo :: Int}

    bar :: Lens' Foo Int
    bar = foo

    makeLenses ''Foo

    Not in scope: ‘foo’ …
        Perhaps you meant one of these:
          data constructor ‘Foo’ (line 1), ‘_foo’ (line 1)

# Using this module in GHCi

You can use `makeLenses` and friends to define lenses right from GHCi, but it's slightly tricky.

First, enable Template Haskell:

    >>> :set -XTemplateHaskell

Then define a bogus type (you can use any name in place of `M`, and you can use the same name many times), and follow the definition by the actual Template Haskell command you want to use:

    >>> data M; makeLenses ''Foo

This will generate lenses for `Foo` and you'll be able to use them from GHCi.

If you want, you can define the type and lenses for it simultaneously with `:{` and `:}`:

    >>> :{
    data Foobar = Foobar {
      _foo :: Int,
      _bar :: Bool }
      deriving (Eq, Show)

    makeLenses ''Foobar
    :}

# `SimpleGetter` and `SimpleFold`

When updates are forbidden (by using `generateUpdateableOptics`), or when a field simply can't be updated (for instance, in the presence of `forall`), instead of `Lens` and `Traversal` we generate `SimpleGetter` and `SimpleFold`. These aren't true `Getter` and `Fold` from lens, so beware. (Still, they're compatible, it's just that you can't do some things with them that you can do with original ones – for instance, `backwards` and `takingWhile` don't work on `SimpleFold`.)

If you want to export true folds, it's recommended that you depend on microlens-contra, use `makeLensesFor` to generate `SimpleFold`s with prefixes, and then export versions of those folds with `fromSimpleFold` applied.

# Generating lenses

makeLenses :: Name -> DecsQ #

Generate lenses for a data type or a newtype.

To use it, you have to enable Template Haskell first:

    {-# LANGUAGE TemplateHaskell #-}

Then, after declaring the datatype (let's say `Foo`), add `makeLenses ''Foo` on a separate line (if you do it before the type is declared, you'll get a “not in scope” error – see the section at the top of this page):

    data Foo = Foo {
      _x :: Int,
      _y :: Bool }

    makeLenses ''Foo

This would generate the following lenses, which can be used to access the fields of `Foo`:

    x :: Lens' Foo Int
    x f foo = (\x' -> foo {_x = x'}) <$> f (_x foo)

    y :: Lens' Foo Bool
    y f foo = (\y' -> foo {_y = y'}) <$> f (_y foo)

(If you don't want a lens to be generated for some field, don't prefix it with “_”.)

If you want to create lenses for many types, you can do it all in one place like this (of course, instead you just can use `makeLenses` several times if you feel it would be more readable):

    data Foo = ...
    data Bar = ...
    data Quux = ...

    concat <$> mapM makeLenses [''Foo, ''Bar, ''Quux]

When the data type has type parameters, it's possible for a lens to do a polymorphic update – i.e. change the type of the thing along with changing the type of the field. For instance, with this type

    data Foo a = Foo {
      _x :: a,
      _y :: Bool }

the following lenses would be generated:

    x :: Lens (Foo a) (Foo b) a b
    y :: Lens' (Foo a) Bool

However, when there are several fields using the same type parameter, type-changing updates are no longer possible:

    data Foo a = Foo {
      _x :: a,
      _y :: a }

generates

    x :: Lens' (Foo a) a
    y :: Lens' (Foo a) a

Finally, when the type has several constructors, some of fields may not be always present – for those, a `Traversal` is generated instead. For instance, in this example `y` can be present or absent:

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

and the following accessors would be generated:

    x :: Lens' FooBar Int
    y :: Traversal' FooBar Bool

So, to get `_y`, you'd have to either use (`^?`) if you're not sure it's there, or (`^?!`) if you're absolutely sure (and if you're wrong, you'll get an exception). Setting and updating `_y` can be done as usual.

makeLensesFor :: [(String, String)] -> Name -> DecsQ #

Like `makeLenses`, but lets you choose your own names for lenses:

    data Foo = Foo {foo :: Int, bar :: Bool}

    makeLensesFor [("foo", "fooLens"), ("bar", "_bar")] ''Foo

would create lenses called `fooLens` and `_bar`. This is useful, for instance, when you don't want to prefix your fields with underscores and want to prefix lenses with underscores instead.

If you give the same name to different fields, it will generate a `Traversal` instead:

    data Foo = Foo {slot1, slot2, slot3 :: Int}

    makeLensesFor [("slot1", "slots"),
                   ("slot2", "slots"),
                   ("slot3", "slots")] ''Foo

would generate

    slots :: Traversal' Foo Int
    slots f foo = Foo <$> f (slot1 foo)
                      <*> f (slot2 foo)
                      <*> f (slot3 foo)

makeLensesWith :: LensRules -> Name -> DecsQ #

Generate lenses with custom parameters.

To see what exactly you can customise, look at the “Configuring lens rules” section. Usually you would build upon the `lensRules` configuration, which is used by `makeLenses`:

    makeLenses = makeLensesWith lensRules

Here's an example of generating lenses that would use lazy patterns:

    data Foo = Foo {_x, _y :: Int}

    makeLensesWith (lensRules & generateLazyPatterns .~ True) ''Foo

When there are several modifications to the rules, the code looks nicer when you use `flip`:

    flip makeLensesWith ''Foo $
      lensRules
        & generateLazyPatterns .~ True
        & generateSignatures   .~ False

makeFields :: Name -> DecsQ #

Generate overloaded lenses.

This lets you deal with several data types having same fields. For instance, let's say you have `Foo` and `Bar`, and both have a field named `x`. To avoid those fields clashing, you would have to use prefixes:

    data Foo a = Foo {
      fooX :: Int,
      fooY :: a }

    data Bar = Bar {
      barX :: Char }

However, if you use `makeFields` on both `Foo` and `Bar` now, it would generate lenses called `x` and `y` – and `x` would be able to access both `fooX` and `barX`! This is done by generating a separate class for each field, and making relevant types instances of that class:

    class HasX s a | s -> a where
      x :: Lens' s a

    instance HasX (Foo a) Int where
      x :: Lens' (Foo a) Int
      x = ...

    instance HasX Bar Char where
      x :: Lens' Bar Char
      x = ...

    class HasY s a | s -> a where
      y :: Lens' s a

    instance HasY (Foo a) a where
      y :: Lens' (Foo a) a
      y = ...

(There's a minor drawback, though: you can't perform type-changing updates with these lenses.)

If you only want to make lenses for some fields, you can prefix them with underscores – the rest would be untouched. If no fields are prefixed with underscores, lenses would be created for all fields.

The prefix must be the same as the name of the name of the data type (not the constructor). If you don't like this behavior, use ``makeLensesWith` `abbreviatedFields`` – it allows any prefix (and even different prefixes).

If you want to use `makeFields` on types declared in different modules, you can do it, but then you would have to export the `Has*` classes from one of the modules – `makeFields` creates a class if it's not in scope yet, so the class must be in scope or else there would be duplicate classes and you would get an “Ambiguous occurrence” error.

Finally, `makeFields` is implemented as ``makeLensesWith` `camelCaseFields``, so you can build on `camelCaseFields` if you want to customise behavior of `makeFields`.

makeClassy :: Name -> DecsQ #

Generate overloaded lenses without ad-hoc classes; useful when there's a collection of fields that you want to make common for several types.

Like `makeFields`, each lens is a member of a class. However, the classes are per-type and not per-field. Let's take the following type:

    data Person = Person {
      _name :: String,
      _age :: Double }

`makeClassy` would generate a single class with 3 methods:

    class HasPerson c where
      person :: Lens' c Person

      age :: Lens' c Double
      age = person.age

      name :: Lens' c String
      name = person.name

And an instance:

    instance HasPerson Person where
      person = id

      name = ...
      age = ...

So, you can use `name` and `age` to refer to the `_name` and `_age` fields, as usual. However, the extra lens – `person` – allows you to do a kind of subtyping. Let's say that there's a type called `Worker` and every worker has the same fields that a person has, but also a `job`. If you were using `makeFields`, you'd do the following:

    data Worker = Worker {
      _workerName :: String,
      _workerAge :: Double,
      _workerJob :: String }

However, with `makeClassy` you can say “every worker is a person” in a more principled way:

    data Worker = Worker {
      _workerPerson :: Person,
      _job :: String }

    makeClassy ''Worker

    instance HasPerson Worker where person = workerPerson

Now you can use `age` and `name` to access name/age of a `Worker`, but you also can use `person` to “downgrade” a `Worker` to a `Person` (and e.g. apply some `Person`-specific function to it).

Unlike `makeFields`, `makeClassy` doesn't make use of prefixes. `_workerPerson` could've just as well been named `_foobar`.

`makeClassy` is implemented as ``makeLensesWith` `classyRules``, so you can build on `classyRules` if you want to customise behavior of `makeClassy`.

# Default lens rules

data LensRules #

Rules used to generate lenses. The fields are intentionally not exported; to create your own rules, see lenses in the “Configuring lens rules” section. You'd have to customise one of the existing rulesets; for an example of doing that, see `makeLensesWith`.

data DefName #

Name to give to a generated lens (used in `lensField`).

Constructors

TopName Name | Simple top-level definiton name
MethodName Name Name | `makeFields`-style class name and method name

#### Instances

Instances details

Show DefName # |
Instance details Defined in Lens.Micro.TH Methods showsPrec :: Int -> DefName -> ShowS # show :: DefName -> String # showList :: [DefName] -> ShowS #
Eq DefName # |
Instance details Defined in Lens.Micro.TH Methods (==) :: DefName -> DefName -> Bool # (/=) :: DefName -> DefName -> Bool #
Ord DefName # |
Instance details Defined in Lens.Micro.TH Methods compare :: DefName -> DefName -> Ordering # (<) :: DefName -> DefName -> Bool # (<=) :: DefName -> DefName -> Bool # (>) :: DefName -> DefName -> Bool # (>=) :: DefName -> DefName -> Bool # max :: DefName -> DefName -> DefName # min :: DefName -> DefName -> DefName #

lensRules :: LensRules #

Lens rules used by default (i.e. in `makeLenses`):

- `generateSignatures` is turned on
- `generateUpdateableOptics` is turned on
- `generateLazyPatterns` is turned off
- `simpleLenses` is turned off
- `lensField` strips “_” off the field name, lowercases the next character after “_”, and skips the field entirely if it doesn't start with “_” (you can see how it's implemented in the docs for `lensField`)
- `lensClass` isn't used (i.e. defined as `const Nothing`)

lensRulesFor #

Arguments

:: [(String, String)] |
    [(fieldName, lensName)]
-> LensRules |

A modification of `lensRules` used by `makeLensesFor` (the only difference is that a simple lookup function is used for `lensField`).

classyRules :: LensRules #

Lens rules used by `makeClassy`:

- `generateSignatures` is turned on
- `generateUpdateableOptics` is turned on
- `generateLazyPatterns` is turned off
- `simpleLenses` is turned on (unlike in `lensRules`)
- `lensField` is the same as in `lensRules`
- `lensClass` just adds “Has” to the name of the type (so for “Person” the generated class would be called “HasPerson” and the type-specific lens in that class would be called “person”)

camelCaseFields :: LensRules #

Lens rules used by `makeFields`:

- `generateSignatures` is turned on
- `generateUpdateableOptics` is turned on
- `generateLazyPatterns` is turned off
- `simpleLenses` is turned on (unlike in `lensRules`)
- `lensField` is more complicated – it takes fields which are prefixed with the name of the type they belong to (e.g. “fooFieldName” for “Foo”), strips that prefix, and generates a class called “HasFieldName” with a single method called “fieldName”. If some fields are prefixed with underscores, underscores would be stripped too, but then fields without underscores won't have any lenses generated for them. Also note that e.g. “foolish” won't have a lens called “lish” generated for it – the prefix must be followed by a capital letter (or else it wouldn't be camel case).
- `lensClass` isn't used (i.e. defined as `const Nothing`)

abbreviatedFields :: LensRules #

Like standard rules used by `makeFields`, but doesn't put any restrictions on the prefix. I.e. if you have fields called

-     _fooBarBaz

-     _someX

-     someY

then the generated lenses would be called `barBaz` and `x`.

# Configuring lens rules

lensField :: Lens' LensRules (Name -> [Name] -> Name -> [DefName]) #

This lets you choose which fields would have lenses generated for them and how would those lenses be called. To do that, you provide a function that would take a field name and output a list (possibly empty) of lenses that should be generated for that field.

Here's the full type of the function you have to provide:

    Name ->     -- The datatype lenses are being generated for
    [Name] ->   -- A list of all fields of the datatype
    Name ->     -- The current field
    [DefName]   -- A list of lens names

Most of the time you won't need the first 2 parameters, but sometimes they are useful – for instance, the list of all fields would be useful if you wanted to implement a slightly more complicated rule like “if some fields are prefixed with underscores, generate lenses for them, but if no fields are prefixed with underscores, generate lenses for all fields”.

As an example, here's a function used by default. It strips “_” off the field name, lowercases the next character after “_”, and skips the field entirely if it doesn't start with “_”:

    \_ _ n ->
      case nameBase n of
        '_':x:xs -> [TopName (mkName (toLower x : xs))]
        _        -> []

You can also generate classes (i.e. what `makeFields` does) by using ``MethodName` className lensName` instead of ``TopName` lensName`.

lensClass :: Lens' LensRules (Name -> Maybe (Name, Name)) #

This lets you choose whether a class would be generated for the type itself (like `makeClassy` does). If so, you can choose the name of the class and the name of the type-specific lens.

For `makeLenses` and `makeFields` this is just `const Nothing`. For `makeClassy` this function is defined like this:

    \n ->
      case nameBase n of
        x:xs -> Just (mkName (Has ++ x:xs), mkName (toLower x : xs))
        []   -> Nothing

createClass :: Lens' LensRules Bool #

Decide whether generation of classes is allowed at all.

If this is disabled, neither `makeFields` nor `makeClassy` would work, regardless of values of `lensField` or `lensClass`. On the other hand, if `lensField` and `lensClass` don't generate any classes, enabling this won't have any effect.

simpleLenses :: Lens' LensRules Bool #

Generate simple (monomorphic) lenses even when type-changing lenses are possible – i.e. `Lens'` instead of `Lens` and `Traversal'` instead of `Traversal`. Just in case, here's an example of a situation when type-changing lenses would be normally generated:

    data Foo a = Foo { _foo :: a }

Generated lens:

    foo :: Lens (Foo a) (Foo b) a b

Generated lens with `simpleLenses` turned on:

    foo :: Lens' (Foo a) a

This option is disabled by default.

generateSignatures :: Lens' LensRules Bool #

Supply type signatures for the generated lenses.

This option is enabled by default. Disable it if you want to write the signature by yourself – for instance, if the signature should be more restricted, or if you want to write haddocks for the lens (as haddocks are attached to the signature and not to the definition).

generateUpdateableOptics :: Lens' LensRules Bool #

Generate “updateable” optics. When turned off, `SimpleFold`s will be generated instead of `Traversal`s and `SimpleGetter`s will be generated instead of `Lens`es.

This option is enabled by default. Disabling it can be useful for types with invariants (also known as “types with smart constructors”) – if you generate updateable optics, anyone would be able to use them to break your invariants.

generateLazyPatterns :: Lens' LensRules Bool #

Generate lenses using lazy pattern matches. This can allow fields of an undefined value to be initialized with lenses:

    data Foo = Foo {_x :: Int, _y :: Bool}
      deriving Show

    makeLensesWith (lensRules & generateLazyPatterns .~ True) ''Foo

    >>> undefined & x .~ 8 & y .~ True
    Foo {_x = 8, _y = True}

(Without `generateLazyPatterns`, the result would be just `undefined`.)

This option is disabled by default. The downside of enabling it is that it can lead to space-leaks and code-size/compile-time increases when lenses are generated for large records.

When you have a lazy lens, you can get a strict lens from it by composing with (`$!`):

    strictLens = ($!) . lazyLens