beginner-friendly-lang/spec.md

## Arithmetic
```
1.2 * 4 + 1
# result: 5.8 
```

## Few types to learn
Most important ones:
- `Num`: big-decimal
- `List[T]`: (linked-) list of `T`
- `Char`: unicode codepoint
- `Unit`: nothing
- `End`: unreachable
- `Cell[T]`: mutable cell of `T`

Advanced (uncommon) types:
- `Int8`, `Int16`, ...
- `Uint8`, ...
- `Flt32`, `Flt64`

## Functions
```
def msg(username: List[Char]) : List[Char] {
  "Hello, " ++ username ++ "!"   # last expr is returned
}

def main() {
  print("hi")
}
main()
```

## Simple, forward type-inference
```
def zero ()  : Flt32 {
  3.1       # error: got Num, but expected F32
}
```

## Bindings
```
let name = "Max"
let passw = "1234"
```

## Mutability only via `ref`
```
let value = Cell.from(1)
print(Num.to_str(!value))     # !cell "observes" the cell
value := 2                    # `:=` mutates the cell
print(Num.to_str(!value))
```

## no confusing function or operator overloading
all operators:
- `Num + Num` (has overloads for fixed width number types)
- `Num - Num` (has overloads for fixed width number types)
- `Num * Num` (has overloads for fixed width number types)
- `Num / Num` (has overloads for fixed width number types)
- `Num ^ Num`: raise to the power (has overloads for fixed width number types)
- `List[t] ++ List[t]`: list concatenation
- `value :: t` (explicitly specify type of value, useful for down-casting structs, or just code readability; does not perform casting)
- `list[index]`
- `!cell`: "observe" a mutable value
- `cell := value`: mutate a mutable value

## non-nominal struct types
```
# `type` creates a non-distinct type alias
type User = { name: List[Char] }
type DbUser = { name: List[Char], pass: List[Char] }

def example(u: User) : DbUser {
  u with pass: "1234"
  # has type { name: List[Char], pass: List[Char] }
}

def example2() : {name: List[Char], pass: List[Char]} { 
  {name:"abc", pass:"123"}
}

def example3() : User {
  example2()     # {name:.., pass:...} can automatically decay to {name:...}
}
```

## (tagged) union types
```
type Option[t] =
  'Err        # If no type specified after tag, defaults to Unit
| 'Some t

# the tags of unions are weakly attached to the types, but won't decay unless they have to
def example(n: Num) : Num {
  let x = 'MyTag n  # type of x is 'MyTag Num
  x                 # tag gets removed because target type is Num
}

def example2(n: Num) : Option[Num] {
  'Some n
}

def example3-invalid() : Option[Num] {
  Unit   # error: can't convert type `Unit` into type `'Err Unit | 'Some Num`
         #        Either label the expression with 'Err,
         #        or change the return type to Option[Unit], and label the expression with 'Some
}

def exampe4(): Option[Num] {
  'Err Unit
  # type of this expression is: `'Err Unit`
  # enums can automatically cast, if all the cases from the source enum also exists in the target enum,
  # which they do here: `'Err Unit` is a case in `'Err Unit | Num`
}

def example5-error(): Option[Num] {
  let x = ( 'Err Unit ) :: Option[Unit]
  x
  # error: can't convert type `'Err Unit | 'Some Unit` into type `'Err Unit | 'Some Num`
  #        The case `'Some Unit` does not exist in the target `'Err Unit | 'Some Num`
}

def example6-error(): Option[Unit] {
  let x = 'Error Unit
  x
  # in this case, the enum tag does not decay, like in `example`,
  # because we are casting to an enum

  # error: can't convert type `'Error Unit` into type `'Err Unit | 'Some Num``
  #        1st possible solution: manually cast to just `Unit` (via `expr :: Unit`), so that it can convert to the second case of the target
  #        2nd possible solution: pattern match against the enum, to rename the tag from 'Error to 'Err
}
```

### pattern 1: labelled arguments
```
def [t] List.remove_prefix(prefix: List[t], list: 'from List[t])
```

## automatic return types
```
def add(a: Num, b: Num) -> _ {
  a + b
}
```

## templated generics
```
def [a,b] add(a: a, b: b) -> _ {
  a + b
}

add(1,2)

add(1,"2")    # error: in template expansion of add[Num,List[Char]]: No definition for `Num + List[Char]`
```

## pattern matching
```
type Option[t] = 'None | 'Some t

def [t] Match.`a++b`(
  # matching against this value
  value: List[t],
  # left hand side of operator
  l: List[t],
  # right hand side of operator
  r: MatchUtil.Var[List[t]]
) -> Option[{ r: List[t] }] {
  match List.remove_prefix(l, 'from value) {
    'Some rem -> 'Some { r: rem }
    'None -> 'None
  }
}
```

then you can do:
```
type Token = 'Public Unit | 'Private Unit | 'Err Unit;
def example(li: List[Char]) -> {t:Token,rem:List[Char]} {
  match li {
    "public" ++ rem  -> {t: 'Public Unit, rem:rem}
    "private" ++ rem -> {t: 'Private Unit, rem:rem}
    _ -> {t: 'Err Unit, rem: li}
  }
}
```

## recursive data types
```
type List[t] = 'End | 'Cons {head:t, tail:List[t]}
# now you might notice an issue with this
# `type` defines non-distinct type alisases
# so what is the type of this...
# Introducing: type self references
# the above example is the same as this:
type List[t] = &a ('End | 'Cons {head:t, tail:a})

# example 2:
# a List[List[t]] is just:
&b ('End | 'Cons {head: &a ('End | 'Cons {head:t, tail:a}), tail: b})
```

Infinitely sized types are not allowed:
```
&a {x:Num, y:a}
```

However, infinite types without size *are* allowed:
```
&a {x:a}
```

This is *not* allowed:
```
&a a
```