From 21661275d8818e7f58afdfdf885a0a2034f09659 Mon Sep 17 00:00:00 2001
From: Ayaz Hafiz <ayaz@rwx.com>
Date: Wed, 20 Apr 2022 14:18:12 -0400
Subject: [PATCH] Show unified variables as a tree

---
 compiler/unify/src/unify.rs | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/compiler/unify/src/unify.rs b/compiler/unify/src/unify.rs
index cebebcca4f..a8ceea4cd6 100644
--- a/compiler/unify/src/unify.rs
+++ b/compiler/unify/src/unify.rs
@@ -275,11 +275,24 @@ pub fn unify_pool(
 }
 
 #[cfg(debug_assertions)]
-fn debug_print_unified_types(subs: &mut Subs, ctx: &Context, before_unified: bool) {
+fn debug_print_unified_types(subs: &mut Subs, ctx: &Context, opt_outcome: Option<&Outcome>) {
+    static mut UNIFICATION_DEPTH: usize = 0;
+
     if std::env::var("ROC_PRINT_UNIFICATIONS").is_ok() {
-        let time = if before_unified { "START" } else { "END" };
-        // if true, print the types that are unified.
-        //
+        let prefix = match opt_outcome {
+            None => "❔",
+            Some(outcome) if outcome.mismatches.is_empty() => "✅",
+            Some(_) => "❌",
+        };
+
+        let depth = unsafe { UNIFICATION_DEPTH };
+        let indent = 2;
+        let (use_depth, new_depth) = if opt_outcome.is_none() {
+            (depth, depth + indent)
+        } else {
+            (depth - indent, depth - indent)
+        };
+
         // NOTE: names are generated here (when creating an error type) and that modifies names
         // generated by pretty_print.rs. So many test will fail with changes in variable names when
         // this block runs.
@@ -294,8 +307,9 @@ fn debug_print_unified_types(subs: &mut Subs, ctx: &Context, before_unified: boo
         let content_2 = subs.get(ctx.second).content;
         let mode = if ctx.mode.is_eq() { "~" } else { "+=" };
         eprintln!(
-            "{}({:?}-{:?}): {:?} {:?} {} {:?} {:?}",
-            time,
+            "{}{}({:?}-{:?}): {:?} {:?} {} {:?} {:?}",
+            " ".repeat(use_depth),
+            prefix,
             ctx.first,
             ctx.second,
             ctx.first,
@@ -304,12 +318,14 @@ fn debug_print_unified_types(subs: &mut Subs, ctx: &Context, before_unified: boo
             ctx.second,
             roc_types::subs::SubsFmtContent(&content_2, subs),
         );
+
+        unsafe { UNIFICATION_DEPTH = new_depth };
     }
 }
 
 fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
     #[cfg(debug_assertions)]
-    debug_print_unified_types(subs, &ctx, true);
+    debug_print_unified_types(subs, &ctx, None);
 
     let result = match &ctx.first_desc.content {
         FlexVar(opt_name) => unify_flex(subs, &ctx, opt_name, None, &ctx.second_desc.content),
@@ -349,7 +365,7 @@ fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
     };
 
     #[cfg(debug_assertions)]
-    debug_print_unified_types(subs, &ctx, false);
+    debug_print_unified_types(subs, &ctx, Some(&result));
 
     result
 }