From fc00cff556372e914d3cf15c39700b4ff7c3541c Mon Sep 17 00:00:00 2001
From: Arnaud Bailly <arnaud@pankzsoft.com>
Date: Sun, 2 Nov 2025 21:35:19 +0100
Subject: better rendering for type errors and evaluation

---
 lambda-calcul/haskell/src/Minilang/Lambda/Infer.hs |  4 ++++
 lambda-calcul/haskell/src/Minilang/Lambda/Unify.hs | 11 +++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

(limited to 'lambda-calcul/haskell/src/Minilang/Lambda')

diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Infer.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Infer.hs
index 2e1814e..ab590c6 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Infer.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Infer.hs
@@ -14,6 +14,7 @@ import GHC.Generics (Generic)
 import qualified Minilang.Lambda.Eval as Eval
 import Minilang.Lambda.Unify (Type (..), UnifyError, apply, unify)
 import Prelude hiding (lookup)
+import qualified Minilang.Lambda.Unify as Unify
 
 data ATerm ann
   = Var Text ann
@@ -25,6 +26,9 @@ data TypeError
   = UnifyError UnifyError
   deriving (Show, Eq)
 
+render :: TypeError -> Text
+render (UnifyError ue) = Unify.render ue
+
 fromTerm :: Eval.Term -> ATerm ()
 fromTerm (Eval.Var x) = Var x ()
 fromTerm (Eval.Lam x body) = Lam x (fromTerm body) ()
diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Unify.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Unify.hs
index f3fff04..96e9990 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Unify.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Unify.hs
@@ -46,10 +46,17 @@ apply subs t =
   foldr (\(x, s) -> subst s x) t subs
 
 data UnifyError
-  = MismatchHead Text Text [Type] [Type]
-  | Circularity Identifier Type
+  = Circularity Identifier Type
   deriving (Show, Eq)
 
+render :: UnifyError -> Text
+render (Circularity x t) =
+  "Cannot unify type variable "
+    <> x
+    <> " with type "
+    <> pretty t
+    <> " due to circularity: The variable already occurs in the type."
+
 unifyOne :: Type -> Type -> Either UnifyError Substitution
 unifyOne (TyVar x) (TyVar y) = if x == y then Right [] else Right [(x, TyVar y)]
 unifyOne (f :-> g) (f' :-> g') =
-- 
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