Safe Haskell | None |
---|---|

Language | Haskell2010 |

The public interface is exposed in CommonMain#Kw

- class Kw (fn :: *) (arg_def :: [*]) r where
- class IsKeyFN (t :: *) (flag :: Bool) | t -> flag
- recToKW :: forall a b. (HMapCxt HList TaggedToKW a b, HConcat b) => Record a -> HList (HConcatR b)
- data K s (c :: *) = K
- data ErrReqdArgNotFound x
- data ErrUnexpectedKW x
- class KWApply f arg_values r where
- class KWApply' flag f arg_values r where
- newtype Arg arg_types arg_values = Arg (HList arg_values)
- reflect_fk :: ReflectFK fn kws => fn -> Arg kws '[]
- class ReflectFK f (kws :: [*])
- class ReflectFK' (flag :: Bool) f kws
- class KW f arg_desc arg_def r where
- class KW' rflag f arg_desc arg_def r where
- class KWAcc arg_desc kw a f arg_def r where
- class KWMerge arg_needed arg_values arg_def f r where
- class KWMerge' kw list atail arg_values arg_def f r where
- class KWMerge'' (flag :: Bool) kw (list :: [*]) atail arg_values arg_def f r where
- class HDelete e (l :: [k]) (l' :: [k])
- class HDelete' (flag :: Bool) e l l'
- data TaggedToKW

# main

class Kw (fn :: *) (arg_def :: [*]) r where Source #

`kw`

takes a `HList`

whose first element is a function, and the rest
of the elements are default values.
A useful trick is to have a final argument `()`

which is not
eaten up by a label (A only takes 1 argument). That way when you supply
the () it knows there are no more arguments (?).

`>>>`

`data A = A`

`>>>`

`instance IsKeyFN (A -> a -> b) True`

`>>>`

`let f A a () = a + 1`

`>>>`

`let f' = f .*. A .*. 1 .*. HNil`

`>>>`

1`kw f' A 0 ()`

`>>>`

2`kw f' ()`

class IsKeyFN (t :: *) (flag :: Bool) | t -> flag Source #

All our keywords must be registered

(~) Bool False flag => IsKeyFN t flag Source # | overlapping/fallback case |

(~) * r (c -> b) => IsKeyFN (K k s c -> r) True Source # | The purpose of this instance is to be able to use the same Symbol
(type-level string) at different types. If they are supposed to be the same,
then use
therefore the following options works:
But you cannot leave off all |

recToKW :: forall a b. (HMapCxt HList TaggedToKW a b, HConcat b) => Record a -> HList (HConcatR b) Source #

convert a `Record`

into a list that can supply
default arguments for `kw`

A bit of setup:

`>>>`

`:set -XQuasiQuotes`

`>>>`

`import Data.HList.RecordPuns`

`>>>`

`let f (_ :: Label "a") a (_ :: Label "b") b () = a `div` b`

`>>>`

`let a = 2; b = 1; f' = f .*. recToKW [pun| a b |]`

`>>>`

2`kw f' ()`

`>>>`

10`kw f' (Label :: Label "a") 10 ()`

## another label type

(~) * r (c -> b) => IsKeyFN (K k s c -> r) True Source # | The purpose of this instance is to be able to use the same Symbol
(type-level string) at different types. If they are supposed to be the same,
then use
therefore the following options works:
But you cannot leave off all |

# types for user error

data ErrReqdArgNotFound x Source #

data ErrUnexpectedKW x Source #

# demo

## setup data types

`>>>`

`:set -XDataKinds -XFlexibleInstances -XMultiParamTypeClasses`

`>>>`

`:set -XScopedTypeVariables -XOverlappingInstances -XTypeFamilies`

`>>>`

`:set -fcontext-stack=100`

We will be using an example inspired by a graphics toolkit -- the area which really benefits from keyword arguments. We first define our labels and useful datatypes

`>>>`

`data Color = Color`

`>>>`

`data Size = Size`

`>>>`

`data Origin = Origin`

`>>>`

`data RaisedBorder = RaisedBorder`

The number of arguments each keyword must be specified by an `IsKeyFN`

instance.

`>>>`

`instance IsKeyFN (Color->a->b) True`

`>>>`

`instance IsKeyFN (Size->a->b) True`

`>>>`

`instance (a ~ (Int,Int)) => IsKeyFN (Origin->a->b) True`

`>>>`

`instance IsKeyFN (RaisedBorder->a->b) True`

Note that if a keyword is always followed by a certain type, that can be specified above using an instance like the one for Origin.

`>>>`

`data CommonColor = Red | Green | Blue deriving Show`

`>>>`

`data RGBColor = RGBColor Int Int Int deriving Show`

and two functions:

`>>>`

let make_square Size n Origin (x0,y0) Color (color::CommonColor) = unwords ["Square:", show (n :: Int), "at", show (x0,y0), show color] ++ "\n" :}`:{`

`>>>`

let make_rect Size (nx,ny) Origin (x0,y0) Color (color::RGBColor) RaisedBorder border = unwords ["Rectangle:", show (nx,ny), "at", show (x0,y0), show color, if border then "raised border" else ""] ++ "\n" :}`:{`

We are not interested in really drawing squares and rectangles
here. Therefore, make_square and make_rect return a String, which we
can regard as a ``command'`

to be passed to a low-level graphics
library. The functions make_square and make_rect are genuine functions
and can be used as such. They are not keyword argument functions, yet,
but they set the stage. These functions can be considered an
`interface`

for the keyword argument functions. We should note that
the functions are polymorphic: for example, `Size`

can be any
showable. We must also emphasize the re-use of the labels: The Color
of a square is the value of the enumerated type CommonColor. OTH, the
color of the rectangle is given as an RGB triple. The sizes of the
square and of the rectangle are specified differently too, the same
label notwithstanding.

Once the user wrote the functions such as make_square and make_rect,
he can _automatically_ convert them to their keyword
alternatives. This transformation is done by a function `kw`

. The user
should pass the positional-argument function (`labeled`

as above),
and an HList of default values for some of the labels. The latter may
be HNil if all keyword arguments are required.

The first example (no defaults)

`>>>`

"Square: 1 at (0,10) Red\n"`kw (make_square .*. HNil) Size (1::Int) Origin (0,10) Color Red :: String`

we can permute the arguments at wish

`>>>`

"Square: 1 at (0,10) Red\n"`kw (make_square .*. HNil) Color Red Size (1::Int) Origin (0,10) :: String`

we can also assign a name to a keyword function, or partially apply it:

`>>>`

case kw (make_square .*. HNil) Color Red of f -> "here: " ++ f Origin (0,10) Size (1::Int) :} "here: Square: 1 at (0,10) Red\n"`:{`

Note that it is necessary to use a monomorphic pattern binding here (lambda or
case). One way to get around this is to pass `f`

instead of `kw f`

around:

`>>>`

let f = hEnd $ hBuild make_square Color Red in "here: " ++ kw f Origin (0,10) Size (1::Int) :} "here: Square: 1 at (0,10) Red\n"`:{`

The following is a more interesting example, with the defaults:

`>>>`

let addDef f = f .*. Origin .*. (0,10) .*. RaisedBorder .*. True .*. HNil in kw (addDef make_square) Size (1::Int) Color Red ++ kw (addDef make_rect) Color (RGBColor 0 10 255) Size (1.0::Float, 2.0::Float) :} "Square: 1 at (0,10) Red\nRectangle: (1.0,2.0) at (0,10) RGBColor 0 10 255 raised border\n"`:{`

The argument RaisedBorder is not given, and so the default value is used. Of course, we can override the default:

`>>>`

let addDef f = f .*. Origin .*. (0,10) .*. RaisedBorder .*. True .*. HNil in case kw (addDef make_square) Color of sq -> case kw (addDef make_rect) of re -> sq Red Size (1::Int) ++ re Color (RGBColor 0 10 255) RaisedBorder False Size (1.0::Float, 2.0::Float) :} "Square: 1 at (0,10) Red\nRectangle: (1.0,2.0) at (0,10) RGBColor 0 10 255 \n"`:{`

We have reshuffled a few arguments, just for fun. As you can see, the
function `kw make_rect defaults' is polyvariadic indeed. We chose to
partially apply `Color`

to the function `kw make_square defaults' --
so that the function `sq`

is positional in its first argument, and
keyword in the rest.

If we omit a required argument, we get a type error:

] testse1 = let f x = kw make_square HNil Color Red x ] in "here: " ++ f Origin (0,10) Couldn't match `ErrReqdArgNotFound Size' against `[Char]' Expected type: ErrReqdArgNotFound Size Inferred type: [Char] ...

The error message seems reasonably clear. Likewise we get an error message if we pass to a keyword function an argument it does not expect:

] testse2 = let f x = kw make_square HNil Color Red x ] in "here: " ++ f Origin (0,10) Size (1::Int) ] RaisedBorder False No instances for (Fail (ErrUnexpectedKW RaisedBorder), KWApply [Char] (HCons RaisedBorder (:*: Bool HNil)) [Char]) arising from use of `f' at ... In the second argument of `(++)', namely `f Origin (0,10) Size (1 :: Int) RaisedBorder False'

The function `kw`

receives the appropriately labeled function (such
as make_square) and the HList with default values. The function `kw`

is polymorphic; the overloading is resolved based on the type of the
user function *and* on the type of its continuation. The continuation
indicates if a keyword argument is forthcoming, or not. In the latter
case, `kw`

checks to see if the supplied defaults can provide the
values of the still missing arguments. We see therefore that a
function type is more than it may appear -- the type of a function is
truly a heterogeneous, type level collection! The function `kw`

traverses that collection, thus performing a limited form of
reflection on Haskell functions.

# Implementation details

One of the key tools of the implementation is `kwapply`

, which applies
a function to a polymorphic collection of that function's arguments.
The order of the arguments in the collection is irrelevant. The
contraption kwapply can handle polymorphic functions with arbitrary
number of labeled arguments.

For example, if we define

f1 Size n = show n f2 Size n Color m = unwords ["size:", show n, "color:", show m] f3 Origin x Color m Size n = unwords ["origin:", show x, "size:", show n, "color:",show m]

then we can run

katest1 = kwapply f1 (Size .*. () .*. HNil) katest11 = kwapply f1 (Size .*. "Large" .*. HNil) katest2 = kwapply f2 (Size .*. (1::Int) .*. Color .*. Red .*. HNil) katest21 = kwapply f2 (Color .*. Red .*. Size .*. (1::Int) .*. HNil) katest3 = kwapply f3 (Size .*. (1::Int) .*. Origin .*. (2.0::Float) .*. Color .*. Red .*. HNil)

class KWApply' flag f arg_values r where Source #

((~) [*] (HAppendListR * tail ((:) * kw ((:) * v ([] *)))) l', HAppendList tail ((:) * kw ((:) * v ([] *))), KWApply f l' r) => KWApply' Bool False f ((:) * kw ((:) * v tail)) r Source # | Rotate the arg list ... |

((~) * v' v, KWApply f' tail r) => KWApply' Bool True (kw -> v -> f') ((:) * kw ((:) * v' tail)) r Source # | |

newtype Arg arg_types arg_values Source #

The datatype Arg below is to maintain the state of keyword
accumulation: which keywords we need, and which keyword and values we
have already got.
arg_types is the phantom HList of keywords that are yet to be satisfied.
arg_values is the `HList (kw .*. kw_value .*. etc)`

of already found keywords and their values.

KWMerge k arg_needed arg_values arg_def f r => KW' Bool False f (Arg k arg_needed arg_values) arg_def r Source # | If the continuation r does not promise any more keyword arguments, apply the defaults |

Show (HList vals) => Show (Arg k tys vals) Source # | |

(HDelete * k kw arg_types arg_types', KW f (Arg [k] arg_types' ((:) * kw ((:) * a arg_values))) arg_def r) => KWAcc (Arg [k] arg_types arg_values) kw a f arg_def r Source # | |

## producing lists from a function's arguments

reflect_fk :: ReflectFK fn kws => fn -> Arg kws '[] Source #

that reflects on a user-supplied function. It converts the *type* of a
user function to a collection of keywords required by that
function. This and the previous contraptions may be used to define an
`extended`

version of some user function that takes more arguments --
without the need to enumerate all arguments of the original
function. We thus infringe on the area of object and module systems.

The rest of the implementation is just to convert `kw fn defaults' into the application of kwapply.

Another key contraption is

class ReflectFK f (kws :: [*]) Source #

Reflection on a function: Given a function, return the type list of its keywords

`>>>`

reflect_fk (undefined::Size->Int->Color->CommonColor->String) :: Arg '[Size, Color] '[]`:t reflect_fk (undefined::Size->Int->Color->CommonColor->String)`

`>>>`

reflect_fk (undefined::Size->Int->()->Int) :: Arg '[Size] '[]`:t reflect_fk (undefined::Size->Int->()->Int)`

class ReflectFK' (flag :: Bool) f kws Source #

## collecting arguments

class KW' rflag f arg_desc arg_def r where Source #

(KWAcc arg_desc kw a f arg_def r, (~) * (kw -> a -> r) kwar) => KW' Bool True f arg_desc arg_def kwar Source # | Otherwise, collect the supplied keyword and its value, and recurse for more: |

KWMerge k arg_needed arg_values arg_def f r => KW' Bool False f (Arg k arg_needed arg_values) arg_def r Source # | If the continuation r does not promise any more keyword arguments, apply the defaults |

class KWAcc arg_desc kw a f arg_def r where Source #

Add the real argument to the Arg structure, and continue

## merging default with supplied arguments

class KWMerge arg_needed arg_values arg_def f r where Source #

Add the needed arguments from arg_def to arg_values and continue with kwapply.

That is, we try to satisfy the missing arguments from the defaults. It will be a type error if some required arguments are missing

class KWMerge'' (flag :: Bool) kw (list :: [*]) atail arg_values arg_def f r where Source #

class HDelete e (l :: [k]) (l' :: [k]) Source #

Delete e from l to yield l' The element e must occur in l

# original introduction

From oleg-at-okmij.org Fri Aug 13 14:58:35 2004 To: haskell@haskell.org Subject: Keyword arguments From: oleg-at-pobox.com Message-ID: <20040813215834.F1FF3AB7E@Adric.metnet.navy.mil> Date: Fri, 13 Aug 2004 14:58:34 -0700 (PDT) Status: OR

We show the Haskell implementation of keyword arguments, which goes well beyond records (e.g., in permitting the re-use of labels). Keyword arguments indeed look just like regular, positional arguments. However, keyword arguments may appear in any order. Furthermore, one may associate defaults with some keywords; the corresponding arguments may then be omitted. It is a type error to omit a required keyword argument. The latter property is in stark contrast with the conventional way of emulating keyword arguments via records. Also in marked contrast with records, keyword labels may be reused throughout the code with no restriction; the same label may be associated with arguments of different types in different functions. Labels of Haskell records may not be re-used. Our solution is essentially equivalent to keyword arguments of DSSSL Scheme or labels of OCaml.

Keyword argument functions are naturally polyvariadic: Haskell does support varargs! Keyword argument functions may be polymorphic. As usual, functions with keyword arguments may be partially applied. On the downside, sometimes one has to specify the type of the return value of the function (if the keyword argument function has no signature -- the latter is the norm, see below) -- provided that the compiler cannot figure the return type out on its own. This is usually only the case when we use keyword functions at the top level (GHCi prompt).

Our solution requires no special extensions to Haskell and works with the existing Haskell compilers; it is tested on GHC 6.0.1. The overlapping instances extension is not necessary (albeit it is convenient).

The gist of our implementation is the realization that the type of a function is a polymorphic collection of its argument types -- a collection that we can traverse. This message thus illustrates a limited form of the reflection on a function.

Our implementation is a trivial extension of the strongly-typed polymorphic open records described in http://homepages.cwi.nl/~ralf/HList/

In fact, the implementation relies on the HList library. To run the code (which this message is), one needs to download the HList library from the above site.

The HList paper discusses the issue of labels in some detail. The paper gives three different representations. One of them needs no overlapping instances and is very portable. In this message, we chose a representation that relies on generic type equality and therefore needs overlapping instances as implemented in GHC. Again, this is merely an outcome of our non-deterministic choice. It should be emphasized that other choices are possible, which do not depend on overlapping instances at all. Please see the HList paper for details.

# todo

`better instances for Symbol`

There isn't a pair `(K2 "Origin" (Int, Int))`

`(K "hi")`

that behaves just like Origin below.
something is possible between constraintkinds. See `Fun`

instance (a ~ (Int,Int)) => IsKeyFN (Origin->a->b) True

`wildcard/catchall`

like in R. This would be a special keyword for keyword args that didn't match.
They would be put in a HList/Record argument like `...`

`investigate first-classness of varargs`

- for whatever reason you can't have
`f = kw fn blah`

and then pass more arguments on to fn. This is bad. It used to work (in the ghc6.0 days and probably up to 6.12). Some convenience functions/operators should be added which do the same thing as:

fn `hAppendList` hBuild a b c d e

# internal for type signature prettiness

data TaggedToKW Source #