FEATURE: allow tuples to have true or false as field names.

this allows selects where the condiotion is a boolean and you can choose
the true or false branch.

docsite/site/content/reference/expressions.md

@@ -212,9 +212,10 @@ Conditionals
 
 UCG supports a limited conditional expression called a select. A select
 expression starts with the `select` keyword and is followed by a an expression
-resolving to a string naming the field to select, an expression resolving to
-the default value, and finally a tuple literal to select the field from. If the
-field selected is not in the tuple then the default value will be used.
+resolving to a string or boolean naming the field to select, an expression
+resolving to the default value, and finally a tuple literal to select the field
+from. If the field selected is not in the tuple then the default value will be
+used.
 
 ```
 let want = "baz";
@@ -230,6 +231,11 @@ select "quack", "quux", {
     baz = "foo",
     fuzz = "bang",
 }; // result will be "quux"
+
+let ifresult = select true, NULL, {
+    true = "true result",
+    false = "false result",
+}; // result will be "true result"
 ```
 
 Macros

integration_tests/select_expressions_test.ucg

@@ -37,4 +37,13 @@ assert |
 |;
 assert |
     condmacro("invalid") == {output = NULL};
+|;
+
+let iflike = select true, "default", {
+    true = "I was true",
+    false = "I was false",
+};
+
+assert |
+    iflike == "I was true";
 |;

src/build/mod.rs

@@ -1132,6 +1132,17 @@ impl<'a> Builder<'a> {
             }
             // Otherwise return the default.
             return self.eval_expr(def_expr);
+        } else if let &Val::Boolean(b) = v.deref() {
+            for &(ref fname, ref val_expr) in fields.iter() {
+                if &fname.fragment == "true" && b {
+                    // Fourth return the result of evaluating that field.
+                    return self.eval_expr(val_expr);
+                } else if &fname.fragment == "false" && !b {
+                    return self.eval_expr(val_expr);
+                }
+            }
+            // Otherwise return the default.
+            return self.eval_expr(def_expr);
         } else {
             return Err(Box::new(error::BuildError::new(
                 format!(

src/parse/mod.rs

@@ -301,7 +301,7 @@ make_fn!(
 make_fn!(
     field_value<SliceIter<Token>, (Token, Expression)>,
     do_each!(
-            field => wrap_err!(either!(match_type!(BAREWORD), match_type!(STR)),
+            field => wrap_err!(either!(match_type!(BOOLEAN), match_type!(BAREWORD), match_type!(STR)),
                                "Field names must be a bareword or a string."),
             _ => punct!("="),
             value => expression,