Hello,
I have created an RFC for proper algebraic data types and structural pattern
matching a la Swift or Rust. It's available here:
<https://github.com/nim-lang/RFCs/issues/525>
I go into much more detail on the RFC itself, but here is an example of code it
would allow you to write:
# An implementation of the simple lambda calculus
# (typechecker and Type union omitted)
type Ident = string
type Expr = ref union
Constant: Term
Variable: tuple[id: Ident]
# Annotation: tuple[expr: Expr, kind: Type]
Abstraction: tuple[param: Ident, fn: Expr]
Application: tuple[fn, arg: Expr]
Conditional: tuple[cond, `if`, `else`: Expr]
type Term = union
Unittype Ident = string
type Expr = ref union
Constant: Term
Variable: tuple[id: Ident]
# Annotation: tuple[expr: Expr, kind: Type]
Abstraction: tuple[param: Ident, fn: Expr]
Application: tuple[fn, arg: Expr]
Conditional: tuple[cond, `if`, `else`: Expr]
type Term = union
Unit
Boolean: bool
Natural: uint
Integer: int
Float: float
String: tuple[len: uint, cap: uint, data: seq[uint]]
Enum: tuple[val: uint, data: seq[Type]]
Struct: Table[Ident, Term]
Function: Expr
var globalSymbolTable = initTable[Ident, Term]()
func execute(ast: Expr): Term =
case ast
of Constant(term):
return term
of Variable(id):
return globalSymbolTable[id]
# other structures of applications + abstractions are invalid
of Application(Abstraction(param, fn), arg):
globalSymbolTable[param] = execute(arg)
return execute(fn)
of Conditional(cond, `if`: ifClause, `else`: elseClause):
if execute(cond) == Boolean(true):
return execute(ifClause)
else:
return execute(elseClause)
else:
echo "Failed to execute AST! Improper structure"
quit()
Integer: int
Float: float
String: tuple[len: uint, cap: uint, data: seq[uint]]
Enum: tuple[val: uint, data: seq[Type]]
Struct: Table[Ident, Term]
Function: Expr
var globalSymbolTable = initTable[Ident, Term]()
func execute(ast: Expr): Term =
case ast
of Constant(term):
return term
of Variable(id):
return globalSymbolTable[id]
# other structures of applications + abstractions are invalid
of Application(Abstraction(param, fn), arg):
globalSymbolTable[param] = execute(arg)
return execute(fn)
of Conditional(cond, `if`: ifClause, `else`: elseClause):
if execute(cond) == Boolean(true):
return execute(ifClause)
else:
return execute(elseClause)
else:
echo "Failed to execute AST! Improper structure"
quit()
Run
Please let me know what you think of this proposal on the Github issue or here
in the forum.
