change syntax for callable

anything that can be applied is now callable, this is the first step for
closure, partial application, and call-by-value

src/albireo.pest

@@ -107,11 +107,11 @@ if_expr     =  {
     "if" ~ if_cond ~ "then" ~ if_branch ~ "else" ~ else_branch
 }
 
-call_function   = _{ identifier }
-call_param_list = _{ expr ~ ("," ~ expr)* }
-call_param      =  { call_param_list? }
-call_expr       =  {
-    call_function ~ "(" ~ call_param ~ ")"
+call_function = _{ identifier
+                   | "(" ~ (if_expr | function) ~ ")" }
+call_param    =  { expr ~ ("," ~ expr)* }
+call_expr     =  {
+    call_function ~ ( "(" ~ call_param? ~ ")" )+
 }
 
 binop = _{

src/albireo.rs

@@ -90,7 +90,7 @@ impl Into<BinaryOp> for &str {
 #[derive(Clone, Debug)]
 pub enum Expression {
     IfClause(Box<Expression>, Box<Expression>, Box<Expression>),
-    Application(/*function name*/Identifier, /*params*/Vec<Box<Expression>>), // function call
+    Application(/*function name*/Box<Expression>, /*params*/Vec<Box<Expression>>), // function call
     Identifier(Identifier),
     Number(u64),
     String(String),
@@ -163,10 +163,18 @@ fn parse_if_expr(parsed: Pair<Rule>) -> Option<Box<Expression>> {
 
 fn parse_call_expr(parsed: Pair<Rule>) -> Option<Box<Expression>> {
     let mut p = parsed.into_inner();
-    let identifier = parse_identifier(p.next()?);
+    let callable = {
+        let caller = p.next()?;
+        match caller.as_rule() {
+            Rule::identifier => Box::new(Expression::Identifier(parse_identifier(caller))),
+            Rule::if_expr => parse_if_expr(caller)?,
+            Rule::function => parse_function_expr(caller)?,
+            _ => unreachable!(),
+        }
+    };
     let parsed_params = p.next()?;
     let params = parsed_params.into_inner().map(parse_expr).collect();
-    Some(Box::new(Expression::Application(identifier, params)))
+    Some(Box::new(Expression::Application(callable, params)))
 }
 
 fn parse_single_expr(parsed: Pair<Rule>) -> Box<Expression> {

test/simple/main.air

@@ -41,4 +41,16 @@ increment : fn number => number = function x is
 is_even : fn number => bool = function x is
   if x % 2 then true else false
 
+; invoke a function by name
 bruh : number = increment(one)
+
+; invoke a lambda function
+bruh1 : number = (function x is x)(1)
+
+; invoke a calculation returns a function
+bruh2 : fn bool => number = function cond is
+  (if cond then
+    function x is x + 1
+  else
+    function x is x + 2
+  )(0)