# Description (My bad opinionated opinions :D)

## 0. [Reason - Why?](reason_why.md)


## 1. Naming

Compiler `dopp` for `doppelganger`  
(also `dopp` command),  

Language `DE[F]-lang` for  
`Dlang [with -- influenced by / inspiration from] Erlang [Flavoured language]`.  

File extension - `*.de`.  
File text code - utf8 without BOM.  



# 2. Syntax and Ideas
### (partial moved to next versions)


## 2.1 toml file for config keys-values

```toml
indent_type = 0
indent_matter = 4
indent_out = 2
```
`indent_type = 0` for whitespaces, 1 for tabs;  
`indent_matter = 4` means we have basic indent (Off-side rule) as 4 whitespaces,  
so we do not got error when next line have 5, 6 or 3, 2 whitespaces etc -  
needs to have more (+- 4) whitespaces for change indent (Off-side rule) for next level.  
`indent_out = 2` means we got 2 whitespaces indents in resu;t files.


## 2.2 do not use any reserved words (keywords) for function definition (like in Erlang)
todo add examples


## 2.3 Pattern Matching that transforms into ```if .. else if .. else ..``` or ```switch .. case```
https://dlang.org/spec/statement.html#SwitchStatement  
https://dlang.org/spec/statement.html#case-range  
https://tour.dlang.org/tour/uk/basics/controlling-flow  
https://dlang.org/spec/expression.html#IsExpression  
todo add examples


## 2.4 Pattern Matching at functions too (like in Erlang)
Function overloading in dlang allows to define the same function but with different versions for different data types:
```
int my_func(int x){
    return x;
}

int my_func(string x){
    return x.length; // or other operation(s) with argument
}
```
that similar to erlang's functions clauses:
```erlang
my_func(X) when is_integer(X) -> X;
my_func(Y) when is_binary(Y) orelse is_list(Y) -> string:length(Y). %% is_list for list string (list chars codes)
```

other example
```erlang
my_func(1) -> 1;
my_func(5) -> 5 * 2;
my_func(9) -> 9 * 3;
my_func(X) -> X * 4.
```
->
```
int my_func(int x){
    if(x == 1){
        return 1;
    }else if(x == 5){
        return x * 2;
    }else if(x == 9){
        return x * 3;
    }else{ // other cases
        return x * 4;
    }
}

// or

int my_func(int x){
    switch (x){
        case 1:
            return 1; // if x == 1
        case 5:
            return x * 2; // if x == 5
        case 9:
            return x * 3; // if x == 9
        default:
            return x * 4; // other cases
    }
}
```
note that compiler can optimise few comparison conditions for ```switch``` better than that in ```if .. else```.  


### 2.4.1 pattern matching on strings
There are Pattern Matching on strings/binaries in Erlang, that looks like:
```
my_func(<<"test", Rest/binary>>) -> true;
my_func(_) -> false.

% or
my_func("test" ++ Rest) -> true;
my_func(_) -> false.
```
similar code in D - dlang:
```
bool my_func(string str){
    if(str.length >= 4 && str[0..4] == "test"){
        return true;
    }
    return false;
}

// also we can get rest

import std.stdio;

bool my_func(string str, out string rest){
    if(str.length >= 4 && str[0..4] == "test"){
        rest = str[4..$];
        return true;
    }
    rest = str;
    return false;
}

void main(){
    string rest;
    writeln(my_func("test123", rest)); // true, rest = "123"
    writeln(rest); // "123"
    writeln(my_func("example", rest)); // false, rest = "example"
}

// also we can to match from string ending
bool has_end_test(string str){
    if(str.length >= 4){
        if(str[$-4 .. $] == "test"){
            return true;
        }
    }
    return false;
}
```
In My Humble Opinion be better to make this syntax similar (laconic) as in Erlang.  
upd. whitespaces count between function name and first argument do not matter (>=1 e.g. " " = [␣]{1, }),  
between first argiment and second (and next) must be at least comma and 1 whitespase (", " = [␣]{0,}[,]{1}[␣]{1,} ).


### 2.4.2 pattern matching examples
```
// left = function type, right = arguments type
bool = string str
has_begin_test "test" ++ _ = true
has_begin_test _ = false

->

bool has_begin_test(string str){
    if(str.length >= 4 && str[0..4] == "test"){
        return true;
    }
    return false;
}
```

```
// left = function type, right = arguments type
bool = string str, out string rest
has_begin_test    "test" ++ rest = true
has_begin_test    _ = false

->

// import std.stdio;

bool has_begin_test(string str, out string rest){
    if(str.length >= 4 && str[0..4] == "test"){
        rest = str[4..$];
        return true;
    }
    rest = str;
    return false;
}

/*
void main(){
    string rest;
    
    writeln( has_begin_test("test123", rest) ); // true, rest = "123"
    writeln(rest); // "123"
    
    writeln( has_begin_test("test", rest) ); // true, rest = ""
    writeln(rest); // ""
    
    writeln( has_begin_test("example", rest) ); // false, rest = "example"
    writeln(rest); // "example"
}
*/
```

```
// function type = arguments type =
//   next lines function body
bool has_end_test = string str =
has_end_test _ ++ "test" = true
has_end_test _ = false

->

// match from string ending
bool has_end_test(string str){
    if(str.length >= 4){
        if(str[$-4 .. $] == "test"){
            return true;
        }
    }
    return false;
}
```

```
// function type = arguments type =
//   next lines function body
bool has_end_test = string str, out string rest =
has_end_test rest ++ "test" = true
has_end_test _ = false

->

// import std.stdio;

bool has_end_test(string str, out string rest){
    if(str.length >= 4 && str[$-4 .. $] == "test"){
        rest = str[0 .. $-4];
        return true;
    }
    rest = str;
    return false;
}

/*
void main(){
    string rest;
    
    writeln( has_end_test("hellotest", rest) ); // true, rest = "hello"
    writeln(rest); // "hello"
    
    writeln( has_end_test("example", rest) ); // false, rest = "example"
    writeln(rest); // "example"
    
    writeln( has_end_test("test", rest) ); // true, rest = ""
    writeln(rest); // ""
}
*/
```

```
// function type = arguments type =
//   next lines function body
my_func int = int x =
my_func 1 = 1
my_func 5 = 5 * 2
my_func 9 = 9 * 3
my_func X = X * 4

->

int my_func(int x){
    if(x == 1){
        return 1;
    }else if(x == 5){
        return 5 * 2; // return x * 2;
    }else if(x == 9){
        return 9 * 3; // return x * 3;
    }else{ // other cases
        return x * 4;
    }
}
```

```
// function type = arguments type =
//   next lines function body
// "_sw" in function name' end for use switch (no "_sw" in dlang function name)
int my_func_sw = int x =
my_func_sw 1 = 1
my_func_sw 5 = 5 * 2
my_func_sw 9 = 9 * 3
my_func_sw X = X * 4

->

int my_func(int x){
    switch(x){
        case 1:
            return 1; // if x == 1
        case 5:
            return 5 * 2; // return x * 2; // if x == 5
        case 9:
            return 9 * 3; // return x * 3; // if x == 9
        default:
            return x * 4; // other cases
    }
}
```

```
// ptn = pattern = for pattern matching like `case X of` in erlang
y = ptn x
  1 = 1
  5 = 5 * 2
  9 = 9 * 3
  _ = x * 4

->

if (x == 1){
    y = 1;
}else if(x == 5){
    y = 5 * 2; // return x * 2;
}else if(x == 9){
    y = 9 * 3; // return x * 3;
}else{ // other cases
    y = x * 4;
}
```

```
// ptns = use switch in dlang for this pattern matching
y = ptns x
  1 = 1
  5 = 5 * 2
  9 = 9 * 3
  _ = x * 4

->

switch(x){
    case 1:
        y = 1; // if x == 1
    case 5:
        y = 5 * 2; // y = x * 2; // if x == 5
    case 9:
        y = 9 * 3; // y = x * 3; // if x == 9
    default:
        y = x * 4; // other cases
}
```

todo more examples -- with switch case dlang



## 2.10 reserved words (keywords) list
including https://dlang.org/spec/lex.html#keywords  
```
abstract alias align asm assert auto 
bool break byte 
case cast catch char class const continue 
dchar debug default delegate deprecated do double 
else enum export extern 
false final finally float for foreach foreach_reverse function 
goto  
if immutable import in inout int interface invariant is 
lazy long 
macro mixin module 
new nothrow null 
out override 
package pragma private protected public pure 
real ref return 
scope shared static struct super switch synchronized 
template this throw true try typeid typeof 
ubyte uint ulong union unittest ushort 
version void 
wchar while with 

__FILE__
__FILE_FULL_PATH__
__FUNCTION__
__LINE__
__MODULE__
__PRETTY_FUNCTION__

__gshared
__parameters
__rvalue
__traits
__vector

__DATE__
__EOF__
__TIME__
__TIMESTAMP__
__VENDOR__
__VERSION__


[deprecated keywords] body cdouble cent cfloat creal delete idouble ifloat ireal ucent


ptn
ptns

_
++

*_sw (function name end)

```
todo

### [todo_v2](todo_v2.md), [todo_v3](todo_v3.md) etc