From 8d10fa406d7e5ad42b004fe1c5550188b86afa12 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ond=C5=99ej=20Hru=C5=A1ka?= <ondra@ondrovo.com>
Date: Thu, 22 Oct 2020 21:55:00 +0200
Subject: [PATCH] Minor "else" improvements

- Do not generate s.never to skip Else branch
- Add Display impl to SourcePosition
- Correct SourcePositions pointing to the end of a parsed expr instead of the beginning
- Remove 'true', 'always', 'false' and 'never' from README.md
---
 README.md                     |  3 +--
 crsn/crsn-sexp/src/error.rs   |  6 ++++++
 crsn/crsn-sexp/src/lib.rs     |  5 +++--
 crsn/src/asm/instr/flatten.rs | 14 ++++++++------
 crsn/src/asm/instr/op.rs      | 12 ++++++++----
 examples/test_cond.csn        | 19 +++++++++++++++++++
 6 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/README.md b/README.md
index 42604ae..e5572db 100644
--- a/README.md
+++ b/README.md
@@ -98,8 +98,7 @@ These keywords (among others) are used in conditional branches to specify flag t
 - `nem`, `nempty` … Not empty
 - `eof` … EOF
 - `neof` … Not EOF
-- `true`, `always`, `else` … Always true
-- `false`, `never` … Always false
+- `else` … Always true, may be used in the last branch
 
 # Syntax
 
diff --git a/crsn/crsn-sexp/src/error.rs b/crsn/crsn-sexp/src/error.rs
index e935717..cb5dea9 100644
--- a/crsn/crsn-sexp/src/error.rs
+++ b/crsn/crsn-sexp/src/error.rs
@@ -19,6 +19,12 @@ pub struct SourcePosition {
     pub index: u32,
 }
 
+impl fmt::Display for SourcePosition {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{}:{}", self.line, self.column)
+    }
+}
+
 /// Since errors are the uncommon case, they're boxed. This keeps the size of
 /// structs down, which helps performance in the common case.
 ///
diff --git a/crsn/crsn-sexp/src/lib.rs b/crsn/crsn-sexp/src/lib.rs
index 9eaf113..50815ab 100644
--- a/crsn/crsn-sexp/src/lib.rs
+++ b/crsn/crsn-sexp/src/lib.rs
@@ -331,10 +331,11 @@ fn parse_sexp(s: &str, pos: &mut usize) -> ERes<Sexp> {
     //trace!("parse_sexp {}", pos);
     zspace(s, pos)?;
     let (c, _) = peek(s, *pos)?;
+    let pos0 = *pos;
     let r = if c == '(' {
-        Ok(Sexp::List(parse_list(s, pos)?, spos(s, *pos)))
+        Ok(Sexp::List(parse_list(s, pos)?, spos(s, pos0)))
     } else {
-        Ok(Sexp::Atom(parse_atom(s, pos)?, spos(s, *pos)))
+        Ok(Sexp::Atom(parse_atom(s, pos)?, spos(s, pos0)))
     };
     zspace(s, pos)?;
     r
diff --git a/crsn/src/asm/instr/flatten.rs b/crsn/src/asm/instr/flatten.rs
index d66721a..5af6a87 100644
--- a/crsn/src/asm/instr/flatten.rs
+++ b/crsn/src/asm/instr/flatten.rs
@@ -50,7 +50,7 @@ impl Flatten for InstrWithBranches {
                 }
 
                 if cnt != branch_count - 1 && cond == Cond::True {
-                    warn!("\"Else\" conditional used in non-final branch at {}:{}", branch.pos().line, branch.pos().column);
+                    warn!("\"Else\" conditional used in non-final branch at {}", branch.pos());
                 }
 
                 let next_lbl = if cnt == branch_count - 1 {
@@ -68,11 +68,13 @@ impl Flatten for InstrWithBranches {
                     // optimization for single-branch conditionals with a single instruction
                     ops.push(Op { cond: Some(cond), pos: pos.clone(), kind: flattened.remove(0).kind });
                 } else {
-                    ops.push(Op {
-                        kind: OpKind::BuiltIn(BuiltinOp::Jump(next_lbl.clone())),
-                        pos: pos.clone(),
-                        cond: Some(!cond),
-                    });
+                    if cond != Cond::True { // evoid emiting `op.never`
+                        ops.push(Op {
+                            kind: OpKind::BuiltIn(BuiltinOp::Jump(next_lbl.clone())),
+                            pos: pos.clone(),
+                            cond: Some(!cond),
+                        });
+                    }
                     ops.extend(flattened);
                 }
 
diff --git a/crsn/src/asm/instr/op.rs b/crsn/src/asm/instr/op.rs
index 61fdf92..b81799f 100644
--- a/crsn/src/asm/instr/op.rs
+++ b/crsn/src/asm/instr/op.rs
@@ -50,11 +50,15 @@ impl OpTrait for Op {
             OpKind::Ext(op) => op.to_sexp()
         };
 
+        // TODO rewrite to be more readable?
         if let Some(cond) = self.cond {
-            if let Sexp::List(items, _) = &mut se {
-                if let Some(Sexp::Atom(Atom::S(s), _)) = &mut items.get_mut(0) {
-                    s.push('.');
-                    s.push_str(&cond.to_string());
+            // "true" is used for "else" branches, it has no effect - just omit it
+            if cond != Cond::True {
+                if let Sexp::List(items, _) = &mut se {
+                    if let Some(Sexp::Atom(Atom::S(s), _)) = &mut items.get_mut(0) {
+                        s.push('.');
+                        s.push_str(&cond.to_string());
+                    }
                 }
             }
         }
diff --git a/examples/test_cond.csn b/examples/test_cond.csn
index fc45bce..30877d8 100644
--- a/examples/test_cond.csn
+++ b/examples/test_cond.csn
@@ -1,4 +1,23 @@
 (
+  ; test else
+
+  (cmp 0 5
+    (eq? (fault))
+    (else? (ld r0 15))
+    (lt? (fault)) ; This should produce a warning
+  )
+  (cmp 15 r0
+    (ne? (fault "else did not run")))
+
+  (ld r0 0)
+
+  (cmp 0 5
+    (lt? (nop))
+    (else? (fault "fallthrough to else"))
+  )
+
+  ;
+
   (ld r8 0)
 
   (ld r0 1