From 130de8cea49c848a06174c61c747d9414a5c71b7 Mon Sep 17 00:00:00 2001
From: Nadrieril
Date: Sun, 16 Feb 2020 19:06:23 +0000
Subject: Start requiring Universe to build a Type

---
 dhall/src/semantics/tck/tyexpr.rs | 117 +++++++++++++++++++++++++++++++-------
 1 file changed, 98 insertions(+), 19 deletions(-)

(limited to 'dhall/src/semantics/tck/tyexpr.rs')

diff --git a/dhall/src/semantics/tck/tyexpr.rs b/dhall/src/semantics/tck/tyexpr.rs
index 4999899..3c47a81 100644
--- a/dhall/src/semantics/tck/tyexpr.rs
+++ b/dhall/src/semantics/tck/tyexpr.rs
@@ -1,8 +1,12 @@
-use crate::error::TypeError;
-use crate::semantics::{Hir, HirKind, NzEnv, TyEnv, Value, ValueKind, VarEnv};
+use crate::error::{ErrorBuilder, TypeError};
+use crate::semantics::{mkerr, Hir, NzEnv, TyEnv, Value, ValueKind, VarEnv};
 use crate::syntax::{Builtin, Const, Span};
 use crate::{NormalizedExpr, ToExprOptions};
 
+/// The type of a type. 0 is `Type`, 1 is `Kind`, etc...
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
+pub(crate) struct Universe(u8);
+
 /// An expression representing a type
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct Type {
@@ -16,12 +20,64 @@ pub(crate) struct TyExpr {
     ty: Type,
 }
 
+impl Universe {
+    pub fn from_const(c: Const) -> Self {
+        Universe(match c {
+            Const::Type => 0,
+            Const::Kind => 1,
+            Const::Sort => 2,
+        })
+    }
+    pub fn as_const(self) -> Option<Const> {
+        match self.0 {
+            0 => Some(Const::Type),
+            1 => Some(Const::Kind),
+            2 => Some(Const::Sort),
+            _ => None,
+        }
+    }
+    pub fn next(self) -> Self {
+        Universe(self.0 + 1)
+    }
+    pub fn previous(self) -> Option<Self> {
+        if self.0 == 0 {
+            None
+        } else {
+            Some(Universe(self.0 - 1))
+        }
+    }
+}
+
 impl Type {
+    pub fn new(val: Value, _u: Universe) -> Self {
+        Type { val }
+    }
     pub fn from_const(c: Const) -> Self {
-        Value::from_const(c).into()
+        Self::new(Value::from_const(c), c.to_universe().next())
     }
     pub fn from_builtin(b: Builtin) -> Self {
-        Value::from_builtin(b).into()
+        use Builtin::*;
+        match b {
+            Bool | Natural | Integer | Double | Text => {}
+            _ => unreachable!("this builtin is not a type: {}", b),
+        }
+
+        Self::new(Value::from_builtin(b), Universe::from_const(Const::Type))
+    }
+
+    /// Get the type of this type
+    // TODO: avoid recomputing so much
+    pub fn ty(&self, env: &TyEnv) -> Result<Option<Const>, TypeError> {
+        Ok(self.to_hir(env.as_varenv()).typecheck(env)?.ty().as_const())
+    }
+    /// Get the type of this type
+    // TODO: avoid recomputing so much
+    pub fn ty_univ(&self, env: &TyEnv) -> Result<Universe, TypeError> {
+        Ok(match self.ty(env)? {
+            Some(c) => c.to_universe(),
+            // TODO: hack, might explode
+            None => Const::Sort.to_universe().next(),
+        })
     }
 
     pub fn to_value(&self) -> Value {
@@ -49,9 +105,9 @@ impl Type {
 }
 
 impl TyExpr {
-    pub fn new(kind: HirKind, ty: Type, span: Span) -> Self {
+    pub fn from_hir(hir: &Hir, ty: Type) -> Self {
         TyExpr {
-            hir: Hir::new(kind, span),
+            hir: hir.clone(),
             ty,
         }
     }
@@ -62,16 +118,6 @@ impl TyExpr {
     pub fn ty(&self) -> &Type {
         &self.ty
     }
-    /// Get the kind (the type of the type) of this value
-    // TODO: avoid recomputing so much
-    pub fn get_kind(&self, env: &TyEnv) -> Result<Option<Const>, TypeError> {
-        Ok(self
-            .ty()
-            .to_hir(env.as_varenv())
-            .typecheck(env)?
-            .ty()
-            .as_const())
-    }
 
     pub fn to_hir(&self) -> Hir {
         self.as_hir().clone()
@@ -79,18 +125,51 @@ impl TyExpr {
     pub fn as_hir(&self) -> &Hir {
         &self.hir
     }
-    /// Converts a value back to the corresponding AST expression.
+    /// Converts a closed expression back to the corresponding AST expression.
     pub fn to_expr(&self, opts: ToExprOptions) -> NormalizedExpr {
         self.as_hir().to_expr(opts)
     }
+    pub fn to_expr_tyenv(&self, env: &TyEnv) -> NormalizedExpr {
+        self.as_hir().to_expr_tyenv(env)
+    }
 
     /// Eval the TyExpr. It will actually get evaluated only as needed on demand.
     pub fn eval(&self, env: impl Into<NzEnv>) -> Value {
         self.as_hir().eval(env.into())
     }
+    pub fn ensure_is_type(&self, env: &TyEnv) -> Result<(), TypeError> {
+        if self.ty().as_const().is_none() {
+            return mkerr(
+                ErrorBuilder::new(format!(
+                    "Expected a type, found: `{}`",
+                    self.to_expr_tyenv(env),
+                ))
+                .span_err(
+                    self.span(),
+                    format!(
+                        "this has type: `{}`",
+                        self.ty().to_expr_tyenv(env)
+                    ),
+                )
+                .help(format!(
+                    "An expression in type position must have type `Type`, \
+                     `Kind` or `Sort`",
+                ))
+                .format(),
+            );
+        }
+        Ok(())
+    }
     /// Evaluate to a Type.
-    pub fn eval_to_type(&self, env: impl Into<NzEnv>) -> Type {
-        self.eval(env).into()
+    pub fn eval_to_type(&self, env: &TyEnv) -> Result<Type, TypeError> {
+        self.ensure_is_type(env)?;
+        Ok(Type::new(
+            self.eval(env),
+            self.ty()
+                .as_const()
+                .expect("case handled in ensure_is_type")
+                .to_universe(),
+        ))
     }
     /// Eval a closed TyExpr (i.e. without free variables). It will actually get evaluated only as
     /// needed on demand.
-- 
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