From ec13c45c8ae0f63fa715178308547cfadd8f8c39 Mon Sep 17 00:00:00 2001
From: Joel Kronqvist <joel.kronqvist@iki.fi>
Date: Tue, 12 Aug 2025 20:15:30 +0300
Subject: feat: pattern matching evaluation in step.rs

---
 src/sexp/case.rs |  2 +-
 src/sexp/step.rs | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/sexp/case.rs b/src/sexp/case.rs
index 71f5007..d78f7ad 100644
--- a/src/sexp/case.rs
+++ b/src/sexp/case.rs
@@ -71,7 +71,7 @@ impl SExp {
 		    .map(|vs| vs.0.into_iter().chain(vs.1).collect())
 	    },
 
-	    _ => todo!(),
+	    _ => None,
 
 	}
     }
diff --git a/src/sexp/step.rs b/src/sexp/step.rs
index fd6be9e..97675fa 100644
--- a/src/sexp/step.rs
+++ b/src/sexp/step.rs
@@ -349,6 +349,35 @@ impl SExp {
                 None => panic!("unreachable as per match guard arm"),
             },
 
+	    // case expressions
+	    SCons(op, l) if scons(op.clone(), l.clone()).check_case().is_some() => {
+		let (scrutinee, patarms) = scons(op, l).check_case().unwrap();
+
+		let scrutinee = match scrutinee {
+		    SCons(q, v) if *q == Atom(Quote) || *q == Atom(Vector) => *v,
+		    t => t,
+		};
+
+		for patarm in patarms {
+		    let (pat, arm) = match patarm {
+			SCons(pat, arm) => Ok((*pat, *arm)),
+			_ => Err("unreachable after type checking".to_string())
+		    }?;
+		    let mut arm = match arm {
+			SCons(x, n) if *n == Atom(Nil) => *x,
+			t => t,
+		    };
+
+		    if let Some(ctx) = scrutinee.matches_pat(&pat) {
+			for (name, value) in ctx {
+			    arm = arm.subst(&name, &value);
+			}
+			return Ok(arm);
+		    }
+		}
+		Err("This should be unreachable after type checking".to_string())
+	    },
+
 
             // op value and a1 .. an values, and b0, b1 .. bn not values
             // ---------------------------------------------------------
-- 
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