Safe Haskell	Safe-Inferred
Language	GHC2021

Generic.Data.MetaParse.Cstr

Description

Definitions for parsing data type constructor names on the type level.

Classically, when doing Generic programming in Haskell that inspects data type metadata such as constructor and record names, we reify these early and do any parsing etc. on the term level. Constant folding should compute much of this at compile time, so performance isn't really a worry. But if you're doing failable operations such as parsing, you can't catch failures at compile time.

This module provides definitions for parsing constructor names on the type level, and is used internally in sum type generics. But wait, how do you write a type-level string parser? That's now feasible-- see the Symparsec library :)

Synopsis

class CstrParser' tag => CstrParser tag where
- type ParseCstr tag (str :: Symbol) :: Either ErrorMessage (CstrParseResult tag)
- type ReifyCstrParseResult tag (x :: CstrParseResult tag) :: Constraint
class CstrParser' tag where
- type CstrParseResult tag :: k
type family ForceGCParse dtName cstr a where ...
data Raw
type ParseCstrTo tag r = forall (x :: CstrParseResult tag). ReifyCstrParseResult tag x => Proxy# x -> r

Documentation

class CstrParser' tag => CstrParser tag Source #

Types defining constructor name parsers.

When defining instances of these two classes, ensure that you place an empty TH splice e.g. $(pure []) between the instances. This is due to a GHC bug.

Associated Types

type ParseCstr tag (str :: Symbol) :: Either ErrorMessage (CstrParseResult tag) Source #

Constructor name parser.

The Symparsec library generates type families that look like this. See Generic.Data.Cstr.Parser.Symparsec for handy definitions.

type ReifyCstrParseResult tag (x :: CstrParseResult tag) :: Constraint Source #

Constraint enabling reification of the parsed type-level constructor name.

For example, you might reify '(a, b) :: (Symbol, Symbol) with (KnownSymbol a, KnownSymbol b).

Instances

Instances details

CstrParser Raw Source #
Instance details Defined in Generic.Data.MetaParse.Cstr Associated Types type ParseCstr Raw str :: Either ErrorMessage (CstrParseResult tag :: Type) Source # type ReifyCstrParseResult Raw x Source #

class CstrParser' tag Source #

Types defining constructor name parsers (inner class).

We're forced to separate this associated type family from the other class due to GHC complaining "type constructor cannot be used here (it is defined and used in the same recursive group)".

When defining instances of these two classes, ensure that you place an empty TH splice e.g. $(pure []) between the instances. This is due to a GHC bug.

Associated Types

type CstrParseResult tag :: k Source #

Result kind of the constructor name parser.

Instances

Instances details

CstrParser' Raw Source #
Instance details Defined in Generic.Data.MetaParse.Cstr Associated Types type CstrParseResult Raw :: k1 Source #

type family ForceGCParse dtName cstr a where ... Source #

Unwrap a generic constructor parse result. Emits a TypeError on parse failure.

Equations

ForceGCParse _ _ (Right a) = a
ForceGCParse dtName cstr (Left e) = TypeError (((((Text "error while parsing " :<>: Text dtName) :<>: Text ".") :<>: Text cstr) :<>: Text ":") :$$: e)

data Raw Source #

Type-level parser tag. Return the string unparsed.

Instances

Instances details

CstrParser Raw Source #
Instance details Defined in Generic.Data.MetaParse.Cstr Associated Types type ParseCstr Raw str :: Either ErrorMessage (CstrParseResult tag :: Type) Source # type ReifyCstrParseResult Raw x Source #
CstrParser' Raw Source #
Instance details Defined in Generic.Data.MetaParse.Cstr Associated Types type CstrParseResult Raw :: k1 Source #
type CstrParseResult Raw Source #
Instance details Defined in Generic.Data.MetaParse.Cstr type CstrParseResult Raw = Symbol
type ParseCstr Raw str Source #
Instance details Defined in Generic.Data.MetaParse.Cstr type ParseCstr Raw str = 'Right str :: Either ErrorMessage Symbol
type ReifyCstrParseResult Raw (str :: CstrParseResult Raw :: Type) Source #
Instance details Defined in Generic.Data.MetaParse.Cstr type ReifyCstrParseResult Raw (str :: CstrParseResult Raw :: Type) = KnownSymbol str

type ParseCstrTo tag r = forall (x :: CstrParseResult tag). ReifyCstrParseResult tag x => Proxy# x -> r Source #

Constructor name parse result demotion function using Proxy#.