Add elif chain to functional syntax

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project does not currently adhere to a particular versioning scheme.
 - Add `IO/sleep` builtin function to sleep for a given amount of seconds as a float. ([#581][gh-581])
 - Add primitive file IO functions `IO/FS/{read, write, seek, open, close}`. ([#573][gh-573])
 - Add encoding/decoding builtin functions `Bytes/{decode_utf8, decode_ascii} String/{encode_ascii, decode_ascii} Utf8/{decode_character, REPLACEMENT_CHARACTER}`. ([#580][gh-580])
+- Add `elif` chains to functional syntax. ([#596][gh-596])
 
 ## [0.2.35] - 2024-06-06
 
@@ -351,4 +352,5 @@ and this project does not currently adhere to a particular versioning scheme.
 [gh-582]: https://github.com/HigherOrderCO/Bend/issues/582
 [gh-583]: https://github.com/HigherOrderCO/Bend/issues/583
 [gh-586]: https://github.com/HigherOrderCO/Bend/issues/586
+[gh-596]: https://github.com/HigherOrderCO/Bend/issues/596
 [Unreleased]: https://github.com/HigherOrderCO/Bend/compare/0.2.35...HEAD

docs/syntax.md

@@ -939,6 +939,20 @@ switch _ = condition {
 }
 ```
 
+It is possible to make if-chains using `elif`:
+
+```rust
+if condition1 {
+  0
+} condition2 {
+  1
+} condition3 {
+  2
+} else {
+  3
+}
+```
+
 ### Bend
 
 Bend can be used to create recursive data structures:

src/fun/parser.rs

@@ -492,22 +492,37 @@ impl<'a> TermParser<'a> {
 
       // If
       if self.try_parse_keyword("if") {
+        let mut chain = Vec::new();
         let cnd = self.parse_term()?;
         self.consume("{")?;
         let thn = self.parse_term()?;
         self.consume("}")?;
+
+        chain.push((cnd, thn));
+
+        self.skip_trivia_inline()?;
+        while self.try_parse_keyword("elif") {
+          let cnd = self.parse_term()?;
+          self.consume("{")?;
+          let thn = self.parse_term()?;
+          self.consume("}")?;
+          self.skip_trivia_inline()?;
+          chain.push((cnd, thn));
+        }
+
         self.consume("else")?;
         self.consume("{")?;
         let els = self.parse_term()?;
         self.consume("}")?;
-        return Ok(Term::Swt {
-          arg: Box::new(cnd),
+        let els = chain.into_iter().rfold(els, |acc, (cnd, thn)| Term::Swt {
           bnd: Some(Name::new("%cond")),
+          arg: Box::new(cnd),
           with_bnd: Vec::new(),
           with_arg: Vec::new(),
           pred: Some(Name::new("%cond-1")),
-          arms: vec![els, thn],
+          arms: vec![acc, thn],
         });
+        return Ok(els);
       }
 
       // Match

src/imp/parser.rs

@@ -583,7 +583,7 @@ impl<'a> PyParser<'a> {
 
     self.consume_indent_exactly(*indent)?;
     let (otherwise, nxt_indent) = self.parse_statement(indent)?;
-    let otherwise = elifs.into_iter().fold(otherwise, |acc, (cond, then)| Stmt::If {
+    let otherwise = elifs.into_iter().rfold(otherwise, |acc, (cond, then)| Stmt::If {
       cond: Box::new(cond),
       then: Box::new(then),
       otherwise: Box::new(acc),

tests/golden_tests/compile_file/elif_fun.bend

@@ -0,0 +1,16 @@
+main =
+  let cond1 = (== 1 2)
+  let cond2 = (< 2 1)
+  let cond3 = (> 3 2)
+  let cond4 = (== 2 2)
+  if cond1 {
+    1
+  } elif cond2 {
+    2
+  } elif cond3 {
+    3
+  } elif cond4 {
+    4
+  } else {
+    0
+  }

tests/snapshots/compile_file__elif.bend.snap

@@ -3,7 +3,7 @@ source: tests/golden_tests.rs
 input_file: tests/golden_tests/compile_file/elif.bend
 ---
 @main = j
-  & $(2 ?(((d (e (?(((b (?(((?((0 (* 2)) a) a) (* (* 3))) (b c)) c)) (* (* (* 4)))) (d (e f))) f))) (* (* (* (* 1))))) (g (h (i j))))) ~ [=0x0000001]
+  & $(2 ?(((?(((?(((?((0 (* 4)) a) a) (* (* 3))) b) b) (* (* (* 2)))) d) d) (* (* (* (* 1))))) (g (h (i j))))) ~ [=0x0000001]
   & $(1 g) ~ [<0x0000002]
   & $(2 h) ~ [>0x0000003]
   & $(2 i) ~ [=0x0000002]

tests/snapshots/compile_file__elif_fun.bend.snap

@@ -0,0 +1,9 @@
+---
+source: tests/golden_tests.rs
+input_file: tests/golden_tests/compile_file/elif_fun.bend
+---
+@main = j
+  & $(2 ?(((?(((?(((?((0 (* 4)) a) a) (* (* 3))) b) b) (* (* (* 2)))) d) d) (* (* (* (* 1))))) (g (h (i j))))) ~ [=0x0000001]
+  & $(1 g) ~ [<0x0000002]
+  & $(2 h) ~ [>0x0000003]
+  & $(2 i) ~ [=0x0000002]