schala/TODO.md

# Immediate TODOs / General Code Cleanup

## Symbols

* the symbol table should probably *only* be for global definitions (maybe rename it to reflect this?)
* dealing with variable lookup w/in functions/closures should probably happen in AST -> ReducedAST
    * b/c that's where we go from a string name to a canonical ID (for e.g. 2nd param in 3rd enclosing scope)

* Old notes on a plan of attack:

1. modify visitor so it can handle scopes
  -this is needed both to handle import scope correctly
  -and also to support making FQSNs aware of function parameters

2. Once FQSNs are aware of function parameters, most of the Rc<String> things in eval.rs can go away

## Parser
* I think I can restructure the parser to get rid of most instances of expect!, at least at the beginning of a rule

## Typechecking

* make a type to represent types rather than relying on string comparisons
* look at https://rickyhan.com/jekyll/update/2018/05/26/hindley-milner-tutorial-rust.html

## General code cleanup
* standardize on an error type that isn't String
* implement a visitor pattern for the use of scope_resolver
  * maybe implement this twice: 1) the value-returning, no-default one in the haoyi blogpost,
  * look at
    * https://gitlab.haskell.org/ghc/ghc/wikis/pattern-synonyms
    * the non-value-returning, default one like in rustc (cf. https://github.com/rust-unofficial/patterns/blob/master/patterns/visitor.md)


# Longer-term Ideas

## Language Syntax

* the `type` declaration should have some kind of GADT-like syntax
* use `let` sigil to indicate a variable in a pattern explicitly:

```
  q is MyStruct(let a, Chrono::Trigga) then {
    // a is in scope here

  }
```

* if you have a pattern-match where one variant has a variable and the other
  lacks it instead of treating this as a type error, promote the bound variable
  to an option type

* what if there was something like React jsx syntas built in? i.e. a way to
  automatically transform some kind of markup into a function call, cf. `<h1
  prop="arg">` ->  h1(prop=arg)

* implement and test open/use statements

* Include extensible scala-style `html"string ${var}"` string interpolations

* A neat idea for pattern matching optimization would be if you could match on
  one of several things in a list
ex:
```
if x {
 is (comp, LHSPat, RHSPat) if comp in ["==, "<"] -> ...
}
```

* Schala should have both currying *and* default arguments!
```
fn a(b: Int, c:Int, d:Int = 1) -> Int
    a(1,2) : Int
    a(1,2,d=2): Int
    a(_,1,3) : Int -> Int
    a(1,2, c=_): Int -> Int
    a(_,_,_) : Int -> Int -> Int -> Int
```

* scoped types - be able to define a quick enum type scoped to a function or other type for
  something, that only is meant to be used as a quick bespoke interface between
  two other things


ex.
```
type enum {
  type enum MySubVariant {
    SubVariant1, SubVariant2, etc.
    }
 Variant1(MySubVariant),
 Variant2(...),
 }
```

* inclusive/exclusive range syntax like .. vs ..=


## Typechecking

* cf. the notation mentioned in the cardelli paper, the debug information for the `typechecking` pass should
    * print the generated type variable for every subexpression in an expression
* think about idris-related ideas of multiple implementations of a type for an interface (+ vs * impl for monoids, for preorder/inorder/postorder for Foldable)
* should have an Idris-like `cast To From` function

* something like the swift `Never` type ( https://nshipster.com/never/ ) in the stdlib


## Compilation
* look into Inkwell for rust LLVM bindings
* https://cranelift.readthedocs.io/en/latest/?badge=latest<Paste>

* look at https://gluon-lang.org/doc/nightly/book/embedding-api.html


# Syntax Playground

## Trying if-syntax again

```
//simple if expr
if x == 10 then "a" else "z"

//complex if expr
if x == 10 then {
  let a = 1
  let b = 2
  a + b
} else {
  55
}

// different comparison ops
if x {
 == 1 then "a"
  .isPrime() then "b"
  else "c"
}

/* for now disallow `if x == { 1 then ... }`, b/c hard to parse

//simple pattern-matching
if x is Person("Ivan", age) then age else 0

//match-block equivalent
if x {
 is Person("Ivan", _) then "Ivan"
 is Person(_, age) if age > 13 then "barmitzvah'd"
 else "foo"
 }
```


## (OLD) Playing around with conditional syntax ideas


- if/match playground

simple if
`if x == 1.0 { "a" } else { "b" }`

one comparison multiple targets:
`if x == { 1.0 -> "a", 2.0 -> "b", else -> "c" }`

different comparison operators/ method calls:
`if x { == 1.0 -> "a", eq NaN -> "n", .hella() -> "h", else -> "z" }`

pattern matching/introducing bindings:
`if alice { .age < 18 -> "18", is Person("Alice", age) -> "${age}", else -> "none" }`

pattern matching w/ if-let:
`if person is Person("Alice", age) { "${age}" } else { "nope" }`

-https://soc.github.io/languages/unified-condition-syntax syntax:

`if <cond-expr>" then <then-expr> else <else-expr>`
`if <half-expr> \n <rest-expr1> then <result1-expr> \n <rest-expr2> then <result-expr2> else <result3-expr>`
-and rest-exprs (or "targets") can have 'is' for pattern-matching, actually so can a full cond-expr

UNIFIED IF EXPRESSIONS FINAL WORK:

basic syntax:

`if_expr := if discriminator '{' (guard_expr)* '}'`
`guard_expr := pattern 'then' block_or_expr'`
`pattern := rhs | is_pattern`
`is_pattern := 'is' ???`
`rhs := expression | ???`


if the only two guard patterns are true and false, then the abbreviated syntax:
`'if' discriminator 'then' block_or_expr 'else' block_or_expr`
can replace `'if' discriminator '{' 'true' 'then' block_or_expr; 'false' 'then' block_or_expr '}'`