Allow aliases with the same name as their verb

Previously, when trying to add an alias that used the same name as its
command, an infinite recursion loop would occur, and the program would
panic after the call stack got too deep. For instance, something like:

  alias grep='grep --color=always'

would cause it to panic.

This patch adds a new field to the Environment struct called 'cs' (for
call stack), which can be used to keep track of how many levels deep
into the Poem::read() function we are in. At the moment, it only allows
going two levels deep, but since it's just a u8 value, this could be
increased in the future. The above example now works correctly, but it
does mean that aliases within aliases are not possible, currently.

1 files changed, 17 insertions, 7 deletions

diff --git a/src/poem/read.rs b/src/poem/read.rs
index da13f76..3c4a720 100644
--- a/src/poem/read.rs
+++ b/src/poem/read.rs
@@ -42,8 +42,12 @@ impl fmt::Display for Mishap {
 /// A [Poem] can add more [Verse]s to itself
 trait Appendable {
     type Type;
-    fn add(&mut self, verse: &mut Self::Type, meter: Rune, env: &Environment)
-        -> Result<(), Mishap>;
+    fn add(
+        &mut self,
+        verse: &mut Self::Type,
+        meter: Rune,
+        env: &mut Environment,
+    ) -> Result<(), Mishap>;
 }
 
 impl Appendable for Poem {
@@ -57,7 +61,7 @@ impl Appendable for Poem {
         &mut self,
         verse: &mut Self::Type,
         meter: Rune,
-        env: &Environment,
+        env: &mut Environment,
     ) -> Result<(), Mishap> {
         if verse.is_empty() {
             return Ok(());
@@ -81,10 +85,16 @@ impl Appendable for Poem {
 
         // Check for aliases
         match env.aliases.get(&verse.verb()) {
-            Some(alias) => {
+            Some(alias) if env.cs == 0 => {
+                // Increase the callstack
+                env.cs = 1;
+
                 // Interpret the alias (could be a complex poem)
                 let mut poem = Poem::read(alias.to_string(), env)?;
 
+                // Decrease the callstack
+                env.cs = 0;
+
                 // Try and get the last verse
                 let lv = match poem.last_mut() {
                     Some(lv) => lv,
@@ -109,7 +119,7 @@ impl Appendable for Poem {
                     self.push(v.clone());
                 }
             }
-            None => {
+            Some(_) | None => {
                 // Push verse(s)
                 self.push(verse.clone());
             }
@@ -125,7 +135,7 @@ impl Appendable for Poem {
 
 /// A [Poem] can parse poetry
 pub trait Readable {
-    fn read(poetry: String, env: &Environment) -> Result<Poem, Mishap>;
+    fn read(poetry: String, env: &mut Environment) -> Result<Poem, Mishap>;
 }
 
 impl Readable for Poem {
@@ -135,7 +145,7 @@ impl Readable for Poem {
     /// machine-runnable [Poem]. If there is a parse error, [Poem::read] may
     /// return a [Mishap]. See [Poem::recite][super::recite] for how each
     /// [Verse] in a [Poem] is called.
-    fn read(poetry: String, env: &Environment) -> Result<Poem, Mishap> {
+    fn read(poetry: String, env: &mut Environment) -> Result<Poem, Mishap> {
         // Get all the characters in the input string as an iterator
         let mut chars = poetry.chars().into_iter();