From 3afaf03e74a7930280c6b682d42b8d53785617a4 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Thu, 25 Mar 2021 13:06:07 +0530
Subject: add checked downcast methods to LispExpr

---
 src/lisp/expr.rs | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 58 insertions(+), 4 deletions(-)

diff --git a/src/lisp/expr.rs b/src/lisp/expr.rs
index 40b39eb..acd3365 100644
--- a/src/lisp/expr.rs
+++ b/src/lisp/expr.rs
@@ -1,7 +1,10 @@
-use std::fmt;
+use std::{convert::TryFrom, fmt};
 
 use crate::app::AppState;
-use crate::lisp::{error::LispError, number::LispNumber};
+use crate::lisp::{
+    error::{EvalError, LispError},
+    number::LispNumber,
+};
 
 #[derive(Clone)]
 pub struct PrimitiveFunc {
@@ -13,13 +16,16 @@ impl PrimitiveFunc {
     pub fn call(&self, args: &[LispExpr], app: &mut AppState) -> Result<LispExpr, LispError> {
         if let Some(arity) = self.arity {
             if args.len() < arity {
-                return Err(LispError::EvalError);
+                return Err(EvalError::ArgumentCount(self.arity.map(|u| u as u32)).into());
             }
         }
         (self.closure)(args, app)
     }
 }
 
+pub type Ident = String;
+pub type BoolLit = bool;
+
 #[derive(Clone)]
 pub struct LispFunction {
     pub params: Vec<String>,
@@ -33,7 +39,7 @@ pub enum LispExpr {
     List(Vec<LispExpr>),
     StringLit(String),
     BoolLit(bool),
-    Ident(String),
+    Ident(Ident),
     PrimitiveFunc(PrimitiveFunc),
     Function(LispFunction),
 
@@ -110,3 +116,51 @@ impl fmt::Display for LispExpr {
         Ok(())
     }
 }
+
+impl TryFrom<LispExpr> for LispNumber {
+    type Error = LispError;
+    fn try_from(value: LispExpr) -> Result<Self, Self::Error> {
+        match value {
+            LispExpr::Number(i) => Ok(i),
+            _ => Err(LispError::Eval(EvalError::TypeMismatch)),
+        }
+    }
+}
+
+impl<'a> TryFrom<&'a LispExpr> for &'a LispNumber {
+    type Error = LispError;
+    fn try_from(value: &'a LispExpr) -> Result<Self, Self::Error> {
+        match value {
+            LispExpr::Number(i) => Ok(i),
+            _ => Err(LispError::Eval(EvalError::TypeMismatch)),
+        }
+    }
+}
+
+impl TryFrom<LispExpr> for Ident {
+    type Error = LispError;
+    fn try_from(value: LispExpr) -> Result<Self, Self::Error> {
+        match value {
+            LispExpr::Ident(i) => Ok(i),
+            _ => Err(LispError::Eval(EvalError::TypeMismatch)),
+        }
+    }
+}
+
+impl TryFrom<LispExpr> for BoolLit {
+    type Error = LispError;
+    fn try_from(value: LispExpr) -> Result<Self, Self::Error> {
+        match value {
+            LispExpr::BoolLit(i) => Ok(i),
+            _ => Err(LispError::Eval(EvalError::TypeMismatch)),
+        }
+    }
+}
+
+pub fn is_ident<E: AsRef<LispExpr>>(expr: E) -> bool {
+    matches!(expr.as_ref(), LispExpr::Ident(_))
+}
+
+pub fn is_number<E: AsRef<LispExpr>>(expr: E) -> bool {
+    matches!(expr.as_ref(), LispExpr::Number(_))
+}
-- 
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