PHI 3 - Explicit Polymorphism

3-explicit-polymorphism/3-explicit-polymorphism.md

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+# Introduction
+
+Currently in Hazel every function has to be assigned a monomorphic type.
+This makes it impossible to write code that operates on more than one type
+of data. We currently have an example that implements `map : (Num -> Num)
+-> [Num] -> [Num]`. By adopting this proposal, Hazel's map function can
+have the type `forall a. forall b. (a -> b) -> [a] -> [b]`, and thereby be useful for
+a larger set of values.
+
+Specifically, we propose extending Hazel with two type constructs
+and two expression constructs, bringing Hazel's type system up to
+a System F with holes than a Simply Typed Lambda Calculus with holes.
+
+# Type Language
+
+We'd have to extend UHTyp.t with `TyVar` and `Forall` cases:
+
+  ```reason
+  /* UHTyp.re */
+
+  type operand =
+  ...
+  | Forall(TPat.t, t);
+  | TyVar(VarErrStatus.t, Var.t)
+  ```
+
+  A type pattern is the part of the language between the `forall` and the `.`
+  in `forall a. <type>`. Type patterns will be very simple to begin with:
+
+  ```reason
+  /* TPat.t */
+  type t =
+  | Hole(MetaVar.t)
+  | Var(Var.t)
+  ```
+
+  <!-- TODO: Talk about how TPat.t's might expand in the future -->
+
+# Expression Language
+
+  ```reason
+  /* UHExp.t */
+  type operand =
+  ...
+  | TyLam(TPat.t, block)
+  | TyArg(UHTyp.t)
+  ```
+
+# Construct actions
+
+  We'll be adding the following to `Construct(shape)`:
+
+  ```reason
+  type shape =
+  ...
+  | SForall
+  | STyArg
+  ```
+
+  `Construct(SForall)` will be triggered when the user types "forall" followed by
+  a space in a type hole. This will add a Forall type with the cursor at the type
+  pattern hole and a hole as the body.
+
+  `Construct(STyArg)` will be triggered when the user types "type" followed by a
+  space in an expression hole. This will add a `STyArg.t` in the expression with a
+  type pattern hole.
+
+# Concrete Syntax / Representation
+
+  Forall's appear within types and look like: `forall a. ?`, where `a` is
+  a TyVar.t.
+
+  Type vars in types appear as a string of lowercase letter without any prefix
+  or constructor.
+
+  Type Arg expressions appear as `(type TYPE)`, where TYPE is any type.
+
+  Type Lambdas are written like a lambda: you start with `\`. Then you write
+  "type" followed by a space which will construct a type lambda. If however, you
+  type `type<space>` into an existing lambda that has a non-empty type
+  annotation, the pattern of the lambda will be put into an error state until
+  either you delete the space following "type" or the type annotation. The final
+  representation is `λ a:type.{ }`.
+
+# Backspace actions
+
+  What happens when you press backspace at the following positions? (`|`
+  represents the cursor.) I don't fully understand how this works visual
+  indicators. Any backspace that does not change the tree or the cursor
+  but only changes the visual indicator is not taken into account here.
+
+## Forall
+
+  * `forall| a. body` or `forall a.| body`
+
+  Either of these remove the `forall a.` prefix and leave just the body.
+
+  * `forall a|. type
+
+  If the type variable is a single character, replace the type variable with a TyPat.Hole,
+  otherwise remove the last character of the type variable.
+
+  * `forall a. type|`
+
+  If the body is not a type hole, this forwards the backspace action onto the
+  body, leaving the forall untouched. However, if the body is a type hole, this
+  removes the whole type, leaving only a type hole.
+
+## Type Args
+
+  * `(type| a)` or `(type a)|`
+
+  The entire expression is replaced with a hole.
+
+  * `(type |a)`
+
+  The cursor is moved to `(type| a)`.
+
+  * `(type a|)`
+
+  Any backspace is treated as a backspace on the HTyp.t, represented by `a` here.
+  It's possible to get a TPat.Hole as a result of this backspace.
+
+## Type Lambdas
+
+  * `λ |a:type. { body }` or `λ ?|:type. { body }` or `λ a:type. { |body }` or `λ a:type. { body }|`
+
+
+  The type lambda is removed, leaving just `|body`
+
+  * `λ a|:type. { body }`
+
+  One character is removed from the type variable "a". If it's only one char,
+  we're left with `λ ?|:type. { body }`
+
+  * `λ a:|type. { body }`
+
+  The cursor is moved back to `λ a|:type. { body }`.
+
+  * `λ a:type|. { body }`
+
+  The type lambda is converted to the normal lambda `λ a:typ|. { body }`.
+  Similarly, typing "d" at this cursor position should render a normal lambda
+  with the type annotation "typed".
+
+  * `λ a:type. { body| }`
+
+  Any backspaces here are forwarded the body unless the body is a hole, in
+  which case, backspace deletes the whole expression, leaving just a hole.
+
+# Movement actions
+
+  The semantics of MoveRight and MoveLeft should be clear based on the cursor
+  positions mentioned in the "Backspace actions" section above.
+
+  <!-- TODO: Talk about what happens with contexts / where we'll need them -->