Add assert function

src/library/mod.rs

@@ -57,6 +57,7 @@ pub fn new() -> Scope {
     std.def_func("circle", circle);
 
     // Utility.
+    std.def_func("assert", assert);
     std.def_func("type", type_);
     std.def_func("repr", repr);
     std.def_func("join", join);

src/library/utility.rs

@@ -4,6 +4,17 @@ use std::str::FromStr;
 use super::*;
 use crate::color::{Color, RgbaColor};
 
+/// `assert`: Ensure that a condition is fulfilled.
+pub fn assert(_: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
+    let Spanned { v, span } = args.expect("condition")?;
+    match v {
+        Value::Bool(true) => {}
+        Value::Bool(false) => bail!(span, "assertion failed"),
+        v => bail!(span, "expected boolean, found {}", v.type_name()),
+    }
+    Ok(Value::None)
+}
+
 /// `type`: The name of a value's type.
 pub fn type_(_: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
     Ok(args.expect::<Value>("value")?.type_name().into())
@@ -86,7 +97,7 @@ pub fn abs(_: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
         Value::Relative(v) => Value::Relative(v.abs()),
         Value::Fractional(v) => Value::Fractional(v.abs()),
         Value::Linear(_) => bail!(span, "cannot take absolute value of a linear"),
-        _ => bail!(span, "expected numeric value"),
+        v => bail!(span, "expected numeric value, found {}", v.type_name()),
     })
 }
 
@@ -157,7 +168,11 @@ pub fn len(_: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
         Value::Str(v) => v.len(),
         Value::Array(v) => v.len(),
         Value::Dict(v) => v.len(),
-        _ => bail!(span, "expected string, array or dictionary"),
+        v => bail!(
+            span,
+            "expected string, array or dictionary, found {}",
+            v.type_name(),
+        ),
     }))
 }
 

tests/typ/utility/basics.typ

@@ -1,6 +1,22 @@
 // Test basic functions.
 // Ref: false
 
+---
+// Test the `assert` function.
+#assert(1 + 1 == 2)
+#assert(2..5 == (2, 3, 4))
+#assert(not false)
+
+---
+// Test failing assertions.
+// Error: 9-15 assertion failed
+#assert(1 == 2)
+
+---
+// Test failing assertions.
+// Error: 9-15 expected boolean, found string
+#assert("true")
+
 ---
 // Test the `type` function.
 #test(type(1), "integer")