Add sh function; Improve type check error output

src/abstract_tree/identifier.rs

@@ -47,9 +47,7 @@ impl AbstractTree for Identifier {
         } else {
             for built_in_function in BUILT_IN_FUNCTIONS {
                 if self.0 == built_in_function.name() {
-                    if let Type::Function { return_type, .. } = built_in_function.r#type() {
-                        return Ok(*return_type);
-                    }
+                    return Ok(built_in_function.r#type());
                 }
             }
 

src/built_in_functions/commands.rs

@@ -0,0 +1,33 @@
+use std::process::Command;
+
+use crate::{BuiltInFunction, Error, Map, Result, Type, Value};
+
+pub struct Sh;
+
+impl BuiltInFunction for Sh {
+    fn name(&self) -> &'static str {
+        "sh"
+    }
+
+    fn run(&self, arguments: &[Value], _context: &Map) -> Result<Value> {
+        Error::expect_argument_amount(self, 1, arguments.len())?;
+
+        let command_text = arguments.first().unwrap().as_string()?;
+        let mut command = Command::new("sh");
+
+        for word in command_text.split(' ') {
+            command.arg(word);
+        }
+
+        let output = command.spawn()?.wait_with_output()?.stdout;
+
+        Ok(Value::String(String::from_utf8(output)?))
+    }
+
+    fn r#type(&self) -> crate::Type {
+        Type::Function {
+            parameter_types: vec![Type::String],
+            return_type: Box::new(Type::String),
+        }
+    }
+}

src/built_in_functions/mod.rs

@@ -3,6 +3,7 @@ use crate::{Map, Result, Type, Value};
 
 mod assert;
 mod collections;
+mod commands;
 mod data_formats;
 mod fs;
 mod network;
@@ -14,10 +15,11 @@ mod r#type;
 ///
 /// This is the public interface to access built-in functions by iterating over
 /// the references it holds.
-pub const BUILT_IN_FUNCTIONS: [&dyn BuiltInFunction; 15] = [
+pub const BUILT_IN_FUNCTIONS: [&dyn BuiltInFunction; 16] = [
     &assert::Assert,
     &assert::AssertEqual,
     &collections::Length,
+    &commands::Sh,
     &data_formats::FromJson,
     &data_formats::ToJson,
     &fs::Read,