# Special form

Special forms are operators, subroutines (and the like) that cannot be expressed in the Erg type system. It is surrounded by ``, but it cannot actually be captured.
Also, types such as `Pattern`, `Body`, and `Conv` appear for convenience, but such types do not exist. Its meaning also depends on the context.

## `=`(pat: Pattern, body: Body) -> NoneType

Assign body to pat as a variable. Raise an error if the variable already exists in the same scope or if it doesn't match pat.
It is also used in record attribute definitions and default arguments.

```python
record = {i = 1; j = 2}
f(x: Int, y = 2) = ...
```

`=` has special behavior when the body is a type or a function.
The variable name on the left side is embedded in the object on the right side.

```python
print! Class() # <class <lambda>>
print! x: Int -> x + 1 # <function <lambda>>
C = Class()
print! c # <class C>
f = x: Int -> x + 1
print! f # <function f>
gx: Int = x + 1
print! g # <function g>
KX: Int = Class(...)
print! K # <kind K>
L = X: Int -> Class(...)
print! L # <kind L>
```

The `=` operator has a return value of "undefined".
Multiple assignments and `=` in functions result in syntax errors.

```python
i = j = 1 # SyntaxError: multiple assignments are not allowed
print!(x=1) # SyntaxError: cannot use `=` in function arguments
# hint: did you mean keyword arguments (`x: 1`)?
if True, do:
    i = 0 # SyntaxError: A block cannot be terminated by an assignment expression
```

## `->`(pat: Pattern, body: Body) -> Func

Generate anonymous functions, function types.

## `=>`(pat: Pattern, body: Body) -> Proc

Generate anonymous procedure, procedure type.

## `.`(obj, attr)

Read attributes of obj.
`x.[y, z]` will return the y and z attributes of x as an array.

## `|>`(obj, c: Callable)

Execute `c(obj)`. `x + y |>.foo()` is the same as `(x + y).foo()`.

### |T: Type|(x: Option T)`?` -> T

Postfix operator. Call `x.unwrap()` and `return` immediately in case of error.

## `:`(x, T)

Declares that object `x` is of type `T`. An error is raised if `x` is not a subtype of `T`.

It can be used for variable declarations or as the right-hand side value of an expression.

```erg
# both are OK
x: Int = 1
y = x: Int
```

## `as`(x, T)

Forces the object `x` to be cast to type `T`. If `x` is not a subtype of `T`, an error is raised.
The difference from `:` is that `(x: T): U` when `x: U; U <: T`, but `(x as T): T`.

## match(obj, *lambdas: Lambda)

For obj, execute lambdas that match the pattern.

```python
match[1, 2, 3]:
  (l: Int) -> log "this is type of Int"
  [[a], b] -> log a, b
  [*a] -> log a
# (one two three)
```

## Del|T: Type|(*x: T) -> NoneType

Delete the variable `x`. However, built-in objects cannot be deleted.

```python
a = 1
Del a # OK

Del True # SyntaxError: cannot delete a built-in object
```

## do(body: Body) -> Func

Generate an anonymous function with no arguments. Syntactic sugar for `() ->`.

## do!(body: Body) -> Proc

Generate an anonymous procedure with no arguments. Syntactic sugar for `() =>`.

## set operator

### `[]`(*objs)

Creates an array from arguments or a dict from optional arguments.

### `{}`(*objs)

Create a set from arguments.

### `{}`(*fields: ((Field, Value); N))

Generate a record.

### `{}`(layout, *names, *preds)

Generates a refinement type.

### `*`

Expand a nested collection. It can also be used for pattern matching.

```python
[x, *y] = [1, 2, 3]
assert x == 1 and y == [2, 3]
assert [x, *y] == [1, 2, 3]
assert [*y, x] == [2, 3, 1]
{x; *yz} = {x = 1; y = 2; z = 3}
assert x == 1 and yz == {y = 2; z = 3}
assert {x; *yz} == {x = 1; y = 2; z = 3}
```

## virtual operator

Operators that cannot be used directly by the user.

### ref|T: Type|(x: T) -> Ref T

Returns an immutable reference to the object.

### ref!|T!: MutType|(x: T!) -> Ref! T!

Returns a mutable reference to a mutable object.