Implement TraceIfFalse type-checking and AST transformation.

This caused me some trouble. In my first approach, I ended up having
  multiple traces because nested values would be evaluated twice; once
  as condition, and once as part of the continuation.

  To prevent this, we can simply evaluate the condition once, and return
  plain True / False boolean as outcome. So this effectively transforms any
  expression:

  ```
  expr
  ```

  as

  ```
  if expr { True } else { trace("...", False) }
  ```

crates/aiken-lang/src/ast.rs

@@ -1017,6 +1017,12 @@ pub enum TraceKind {
     Error,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Tracing {
+    NoTraces,
+    KeepTraces,
+}
+
 #[derive(Copy, Clone, PartialEq, Eq)]
 pub struct Span {
     pub start: usize,

crates/aiken-lang/src/tests/check.rs

@@ -1,5 +1,5 @@
 use crate::{
-    ast::{ModuleKind, TypedModule, UntypedModule},
+    ast::{ModuleKind, Tracing, TypedModule, UntypedModule},
     builtins, parser,
     tipo::error::{Error, Warning},
     IdGenerator,
@@ -24,7 +24,14 @@ fn check_module(
     module_types.insert("aiken".to_string(), builtins::prelude(&id_gen));
     module_types.insert("aiken/builtin".to_string(), builtins::plutus(&id_gen));
 
-    let result = ast.infer(&id_gen, kind, "test/project", &module_types, &mut warnings);
+    let result = ast.infer(
+        &id_gen,
+        kind,
+        "test/project",
+        &module_types,
+        Tracing::KeepTraces,
+        &mut warnings,
+    );
 
     result
         .map(|o| (warnings.clone(), o))

crates/aiken-lang/src/tipo/expr.rs

@@ -5,13 +5,14 @@ use vec1::Vec1;
 use crate::{
     ast::{
         Annotation, Arg, ArgName, AssignmentKind, BinOp, CallArg, Clause, ClauseGuard, Constant,
-        RecordUpdateSpread, Span, TraceKind, TypedArg, TypedCallArg, TypedClause, TypedClauseGuard,
-        TypedConstant, TypedIfBranch, TypedMultiPattern, TypedRecordUpdateArg, UnOp, UntypedArg,
-        UntypedClause, UntypedClauseGuard, UntypedConstant, UntypedIfBranch, UntypedMultiPattern,
-        UntypedPattern, UntypedRecordUpdateArg,
+        IfBranch, RecordUpdateSpread, Span, TraceKind, Tracing, TypedArg, TypedCallArg,
+        TypedClause, TypedClauseGuard, TypedConstant, TypedIfBranch, TypedMultiPattern,
+        TypedRecordUpdateArg, UnOp, UntypedArg, UntypedClause, UntypedClauseGuard, UntypedConstant,
+        UntypedIfBranch, UntypedMultiPattern, UntypedPattern, UntypedRecordUpdateArg,
     },
     builtins::{bool, byte_array, function, int, list, string, tuple},
     expr::{TypedExpr, UntypedExpr},
+    format,
     tipo::fields::FieldMap,
 };
 
@@ -29,6 +30,10 @@ use super::{
 pub(crate) struct ExprTyper<'a, 'b> {
     pub(crate) environment: &'a mut Environment<'b>,
 
+    // We tweak the tracing behavior during type-check. Traces are either kept or left out of the
+    // typed AST depending on this setting.
+    pub(crate) tracing: Tracing,
+
     // Type hydrator for creating types from annotations
     pub(crate) hydrator: Hydrator,
 
@@ -377,6 +382,78 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         }
     }
 
+    fn infer_trace_if_false(
+        &mut self,
+        value: UntypedExpr,
+        location: Span,
+    ) -> Result<TypedExpr, Error> {
+        let var_true = TypedExpr::Var {
+            location,
+            name: "True".to_string(),
+            constructor: ValueConstructor {
+                public: true,
+                variant: ValueConstructorVariant::Record {
+                    name: "True".to_string(),
+                    arity: 0,
+                    field_map: None,
+                    location: Span::empty(),
+                    module: String::new(),
+                    constructors_count: 2,
+                },
+                tipo: bool(),
+            },
+        };
+
+        let var_false = TypedExpr::Var {
+            location,
+            name: "False".to_string(),
+            constructor: ValueConstructor {
+                public: true,
+                variant: ValueConstructorVariant::Record {
+                    name: "False".to_string(),
+                    arity: 0,
+                    field_map: None,
+                    location: Span::empty(),
+                    module: String::new(),
+                    constructors_count: 2,
+                },
+                tipo: bool(),
+            },
+        };
+
+        let text = TypedExpr::String {
+            location,
+            tipo: string(),
+            value: format!(
+                "{} ? False",
+                format::Formatter::new().expr(&value).to_pretty_string(999)
+            ),
+        };
+
+        let typed_value = self.infer(value)?;
+
+        self.unify(typed_value.tipo(), bool(), typed_value.location(), false)?;
+
+        match self.tracing {
+            Tracing::NoTraces => Ok(typed_value),
+            Tracing::KeepTraces => Ok(TypedExpr::If {
+                location,
+                branches: vec1::vec1![IfBranch {
+                    condition: typed_value,
+                    body: var_true,
+                    location,
+                }],
+                final_else: Box::new(TypedExpr::Trace {
+                    location,
+                    tipo: bool(),
+                    text: Box::new(text),
+                    then: Box::new(var_false),
+                }),
+                tipo: bool(),
+            }),
+        }
+    }
+
     fn infer_binop(
         &mut self,
         name: BinOp,
@@ -1879,12 +1956,15 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             })
         }
 
-        Ok(TypedExpr::Trace {
+        match self.tracing {
+            Tracing::NoTraces => Ok(then),
+            Tracing::KeepTraces => Ok(TypedExpr::Trace {
                 location,
                 tipo,
                 then: Box::new(then),
                 text: Box::new(text),
-        })
+            }),
+        }
     }
 
     fn infer_value_constructor(
@@ -2058,7 +2138,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         self.environment.instantiate(t, ids, &self.hydrator)
     }
 
-    pub fn new(environment: &'a mut Environment<'b>) -> Self {
+    pub fn new(environment: &'a mut Environment<'b>, tracing: Tracing) -> Self {
         let mut hydrator = Hydrator::new();
 
         hydrator.permit_holes(true);
@@ -2066,6 +2146,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         Self {
             hydrator,
             environment,
+            tracing,
             ungeneralised_function_used: false,
         }
     }

crates/aiken-lang/src/tipo/infer.rs

@@ -3,8 +3,8 @@ use std::collections::HashMap;
 use crate::{
     ast::{
         DataType, Definition, Function, Layer, ModuleConstant, ModuleKind, RecordConstructor,
-        RecordConstructorArg, Span, TypeAlias, TypedDefinition, TypedModule, UntypedDefinition,
-        UntypedModule, Use, Validator,
+        RecordConstructorArg, Span, Tracing, TypeAlias, TypedDefinition, TypedModule,
+        UntypedDefinition, UntypedModule, Use, Validator,
     },
     builtins,
     builtins::function,
@@ -29,6 +29,7 @@ impl UntypedModule {
         kind: ModuleKind,
         package: &str,
         modules: &HashMap<String, TypeInfo>,
+        tracing: Tracing,
         warnings: &mut Vec<Warning>,
     ) -> Result<TypedModule, Error> {
         let name = self.name.clone();
@@ -83,7 +84,8 @@ impl UntypedModule {
         }
 
         for def in consts.into_iter().chain(not_consts) {
-            let definition = infer_definition(def, &name, &mut hydrators, &mut environment, kind)?;
+            let definition =
+                infer_definition(def, &name, &mut hydrators, &mut environment, tracing, kind)?;
             definitions.push(definition);
         }
 
@@ -148,6 +150,7 @@ fn infer_definition(
     module_name: &String,
     hydrators: &mut HashMap<String, Hydrator>,
     environment: &mut Environment<'_>,
+    tracing: Tracing,
     kind: ModuleKind,
 ) -> Result<TypedDefinition, Error> {
     match def {
@@ -192,7 +195,7 @@ fn infer_definition(
                         .map(|(arg_name, tipo)| arg_name.set_type(tipo.clone()))
                         .collect();
 
-                    let mut expr_typer = ExprTyper::new(environment);
+                    let mut expr_typer = ExprTyper::new(environment, tracing);
 
                     expr_typer.hydrator = hydrators
                         .remove(&name)
@@ -266,6 +269,7 @@ fn infer_definition(
                 module_name,
                 hydrators,
                 environment,
+                tracing,
                 kind,
             )? {
                 if !typed_fun.return_type.is_bool() {
@@ -297,9 +301,14 @@ fn infer_definition(
         }
 
         Definition::Test(f) => {
-            if let Definition::Fn(f) =
-                infer_definition(Definition::Fn(f), module_name, hydrators, environment, kind)?
-            {
+            if let Definition::Fn(f) = infer_definition(
+                Definition::Fn(f),
+                module_name,
+                hydrators,
+                environment,
+                tracing,
+                kind,
+            )? {
                 environment.unify(f.return_type.clone(), builtins::bool(), f.location, false)?;
 
                 Ok(Definition::Test(f))
@@ -505,7 +514,8 @@ fn infer_definition(
                 })
             }
 
-            let typed_expr = ExprTyper::new(environment).infer_const(&annotation, *value)?;
+            let typed_expr =
+                ExprTyper::new(environment, tracing).infer_const(&annotation, *value)?;
 
             let tipo = typed_expr.tipo();
 

crates/aiken-project/src/blueprint/validator.rs

@@ -177,7 +177,7 @@ mod test {
     use crate::{module::ParsedModule, PackageName};
     use aiken_lang::{
         self,
-        ast::{ModuleKind, TypedDataType, TypedFunction},
+        ast::{ModuleKind, Tracing, TypedDataType, TypedFunction},
         builder::{DataTypeKey, FunctionAccessKey},
         builtins, parser,
         tipo::TypeInfo,
@@ -253,6 +253,7 @@ mod test {
                     module.kind,
                     &self.package.to_string(),
                     &self.module_types,
+                    Tracing::NoTraces,
                     &mut warnings,
                 )
                 .expect("Failed to type-check module");

crates/aiken-project/src/lib.rs

@@ -14,7 +14,7 @@ pub mod telemetry;
 
 use crate::blueprint::{schema::Schema, Blueprint};
 use aiken_lang::{
-    ast::{Definition, Function, ModuleKind, TypedDataType, TypedFunction},
+    ast::{Definition, Function, ModuleKind, Tracing, TypedDataType, TypedFunction},
     builder::{DataTypeKey, FunctionAccessKey},
     builtins,
     tipo::TypeInfo,
@@ -496,6 +496,8 @@ where
                         kind,
                         &self.config.name.to_string(),
                         &self.module_types,
+                        // TODO: Make configurable
+                        Tracing::KeepTraces,
                         &mut type_warnings,
                     )
                     .map_err(|error| Error::Type {