Fcf.Utils · first-class-families-0.8.2.0

type Eval (Constraints '[]) = ()Defined in first-class-families-0.8.2.0 · Fcf.Utils
type Eval (Constraints (a ': as)) = (a, Eval (Constraints as))Defined in first-class-families-0.8.2.0 · Fcf.Utils

typetype Else = 'Else_

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datadata Error (b :: Symbol) (c :: a)

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Instances1Eval

type Eval (Error msg) = TypeError ('Text msg)Defined in first-class-families-0.8.2.0 · Fcf.Utils

typetype Is = 'Is_

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classclass IsBool (b :: Bool) where

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Methods

_If :: (b ~ 'True => r) -> (b ~ 'False => r) -> r

Instances2IsBool

IsBool 'FalseDefined in first-class-families-0.8.2.0 · Fcf.Utils
IsBool 'TrueDefined in first-class-families-0.8.2.0 · Fcf.Utils

datadata Match j k

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familytype family Stuck :: a

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datadata TError (b :: ErrorMessage) (c :: a)

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Instances1Eval

type Eval (TError msg) = TypeError msgDefined in first-class-families-0.8.2.0 · Fcf.Utils

datadata TyEq (c :: a) (d :: b) (e :: Bool)

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Instances1Eval

type Eval (TyEq a b) = TyEqImpl a bDefined in first-class-families-0.8.2.0 · Fcf.Utils

familytype family If (cond :: Bool) (tru :: k) (fls :: k) :: k where

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Type-level If. If True a b ==> a; If False a b ==> b

Equations

If 'True tru fls = tru
If 'False tru fls = fls

datadata ErrorMessage

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A description of a custom type error.

Constructors

Text Symbol
Show the text as is.
forall t. ShowType t
Pretty print the type. ShowType :: k -> ErrorMessage
ErrorMessage :<>: ErrorMessageinfixl 6
Put two pieces of error message next to each other.
ErrorMessage :$$: ErrorMessageinfixl 5
Stack two pieces of error message on top of each other.

familytype family TypeError (a :: ErrorMessage) :: b where

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The type-level equivalent of error.

The polymorphic kind of this type allows it to be used in several settings. For instance, it can be used as a constraint, e.g. to provide a better error message for a non-existent instance,

-- in a context
instance TypeError (Text "Cannot Show functions." :$$:
                    Text "Perhaps there is a missing argument?")
      => Show (a -> b) where
    showsPrec = error "unreachable"

It can also be placed on the right-hand side of a type-level function to provide an error for an invalid case,

type family ByteSize x where
   ByteSize Word16   = 2
   ByteSize Word8    = 1
   ByteSize a        = TypeError (Text "The type " :<>: ShowType a :<>:
                                  Text " is not exportable.")