diff --git a/src/foreign_macros/atomic_impl.rs b/src/foreign_macros/atomic_impl.rs
index bb6a7c7..d77196d 100644
--- a/src/foreign_macros/atomic_impl.rs
+++ b/src/foreign_macros/atomic_impl.rs
@@ -65,7 +65,7 @@ use crate::Primitive;
 #[macro_export]
 macro_rules! atomic_impl {
   ($typ:ident) => {
-    atomic_impl! {$typ, |this: &Self, _: $crate::interpreter::Context| {
+    $crate::atomic_impl! {$typ, |this: &Self, _: $crate::interpreter::Context| {
       use $crate::foreign::ExternFn;
       Ok(this.clone().to_xfn_cls())
     }}
diff --git a/src/foreign_macros/externfn_impl.rs b/src/foreign_macros/externfn_impl.rs
index 81c2b9b..49165f6 100644
--- a/src/foreign_macros/externfn_impl.rs
+++ b/src/foreign_macros/externfn_impl.rs
@@ -19,7 +19,7 @@ use crate::{atomic_impl, atomic_redirect};
 /// reference to self and a closure. This can be used in conjunction with
 /// [atomic_impl] and [atomic_redirect] to normalize the argument automatically
 /// before using it.
-/// 
+///
 /// See [atomic_impl] for an example.
 #[macro_export]
 macro_rules! externfn_impl {
diff --git a/src/foreign_macros/mod.rs b/src/foreign_macros/mod.rs
index fb8c37e..8355c6e 100644
--- a/src/foreign_macros/mod.rs
+++ b/src/foreign_macros/mod.rs
@@ -3,3 +3,4 @@ mod atomic_impl;
 mod atomic_inert;
 mod atomic_redirect;
 mod externfn_impl;
+mod write_fn_step;
diff --git a/src/foreign_macros/write_fn_step.rs b/src/foreign_macros/write_fn_step.rs
new file mode 100644
index 0000000..1f6e1e7
--- /dev/null
+++ b/src/foreign_macros/write_fn_step.rs
@@ -0,0 +1,125 @@
+#[allow(unused)] // for doc
+use crate::foreign::ExternFn;
+#[allow(unused)] // for doc
+use crate::interpreted::ExprInst;
+
+/// Write one step in the state machine representing a simple n-ary non-variadic
+/// Orchid function.
+///
+/// There are three ways to call this macro for the initial state, internal
+/// state, and exit state. All of them are demonstrated in one example and
+/// discussed below.
+///
+/// ```
+/// use orchidlang::{write_fn_step, Literal, Primitive};
+/// use orchidlang::interpreted::Clause;
+/// use orchidlang::stl::litconv::{with_str, with_uint};
+/// use orchidlang::stl::RuntimeError;
+///
+/// // Initial state
+/// write_fn_step!(pub CharAt2 > CharAt1);
+/// // Middle state
+/// write_fn_step!(
+///   CharAt1 {}
+///   CharAt0 where s = |x| with_str(x, |s| Ok(s.clone()))
+/// );
+/// // Exit state
+/// write_fn_step!(
+///   CharAt0 { s: String }
+///   i = |x| with_uint(x, Ok)
+///   => {
+///     if let Some(c) = s.chars().nth(i as usize) {
+///       Ok(Clause::P(Primitive::Literal(Literal::Char(c))))
+///     } else {
+///       RuntimeError::fail(
+///         "Character index out of bounds".to_string(),
+///         "indexing string",
+///       )?
+///     }
+///   }
+/// );
+/// ```
+///
+/// The initial state simply defines an empty marker struct and implements
+/// [ExternFn] on it, transitioning into a new struct which is assumed to have a
+/// single field called `expr_inst` of type [ExprInst].
+///
+/// The middle state defines a sequence of arguments with types similarly to a
+/// struct definition. A field called `expr_inst` of type [ExprInst] is added
+/// implicitly, so the first middle state has an empty field list. The next
+/// state is also provided, alongside the name and conversion function of the
+/// next parameter which is [FnOnce(&ExprInst) -> Result<_, RuntimeError>]. The
+/// success type is inferred from the type of the field at the place of its
+/// actual definition. This conversion is done in the implementation of
+/// [ExternFn] which also places the new [ExprInst] into `expr_inst` on the next
+/// state.
+///
+/// The final state defines the sequence of all arguments except for the last
+/// one with the same syntax used by the middle state, and the name and
+/// conversion lambda of the final argument without specifying the type - it is
+/// to be inferred. This state also specifies the operation that gets executed
+/// when all the arguments are collected. Uniquely, this "function body" isn't
+/// specified as a lambda but rather as an expression invoked with all the
+/// argument names bound. The arguments here are all references to their actual
+/// types except for the last one which is converted from [ExprInst] immediately
+/// before the body is evaluated.
+#[macro_export]
+macro_rules! write_fn_step {
+  ($quant:vis $name:ident > $next:ident) => {
+    #[derive(Clone)]
+    $quant struct $name;
+    $crate::externfn_impl!{
+      $name,
+      |_: &Self, expr_inst: $crate::interpreted::ExprInst| {
+        Ok($next{ expr_inst })
+      }
+    }
+  };
+  (
+    $quant:vis $name:ident {
+      $( $arg:ident : $typ:ty ),*
+    }
+    $next:ident where $added:ident = $extract:expr
+  ) => {
+    #[derive(std::fmt::Debug, Clone)]
+    $quant struct $name {
+      $( $arg: $typ, )*
+      expr_inst: $crate::interpreted::ExprInst,
+    }
+    $crate::atomic_redirect!($name, expr_inst);
+    $crate::atomic_impl!($name);
+    $crate::externfn_impl!(
+      $name,
+      |this: &Self, expr_inst: $crate::interpreted::ExprInst| {
+        let lambda = $extract;
+        Ok($next{
+          $( $arg: this.$arg.clone(), )*
+          $added: lambda(&this.expr_inst)?,
+          expr_inst
+        })
+      }
+    );
+  };
+  (
+    $quant:vis $name:ident {
+      $( $arg:ident: $typ:ty ),*
+    }
+    $added:ident = $extract:expr
+    => $process:expr
+  ) => {
+    #[derive(std::fmt::Debug, Clone)]
+    $quant struct $name {
+      $( $arg: $typ, )+
+      expr_inst: $crate::interpreted::ExprInst,
+    }
+    $crate::atomic_redirect!($name, expr_inst);
+    $crate::atomic_impl!(
+      $name,
+      |Self{ $($arg, )* expr_inst }: &Self, _| {
+        let lambda = $extract;
+        let $added = lambda(expr_inst)?;
+        $process
+      }
+    );
+  };
+}
diff --git a/src/stl/assertion_error.rs b/src/stl/assertion_error.rs
index efad123..6046a22 100644
--- a/src/stl/assertion_error.rs
+++ b/src/stl/assertion_error.rs
@@ -8,11 +8,13 @@ use crate::representations::interpreted::ExprInst;
 /// hold.
 #[derive(Clone)]
 pub struct AssertionError {
-  pub value: ExprInst,
-  pub assertion: &'static str,
+  value: ExprInst,
+  assertion: &'static str,
 }
 
 impl AssertionError {
+  /// Construct, upcast and wrap in a Result that never succeeds for easy
+  /// short-circuiting
   pub fn fail<T>(
     value: ExprInst,
     assertion: &'static str,
@@ -20,6 +22,7 @@ impl AssertionError {
     return Err(Self { value, assertion }.into_extern());
   }
 
+  /// Construct and upcast to [ExternError]
   pub fn ext(value: ExprInst, assertion: &'static str) -> Rc<dyn ExternError> {
     return Self { value, assertion }.into_extern();
   }
diff --git a/src/stl/litconv.rs b/src/stl/litconv.rs
index 37ec1e0..6cc185e 100644
--- a/src/stl/litconv.rs
+++ b/src/stl/litconv.rs
@@ -1,3 +1,5 @@
+//! Utility functions that operate on literals. Because of the parallel locked
+//! nature of [ExprInst], returning a reference to [Literal] is not possible.
 use std::rc::Rc;
 
 use super::assertion_error::AssertionError;
diff --git a/src/stl/mod.rs b/src/stl/mod.rs
index 63e77fa..2233af6 100644
--- a/src/stl/mod.rs
+++ b/src/stl/mod.rs
@@ -3,14 +3,16 @@ mod assertion_error;
 mod bool;
 mod conv;
 mod cpsio;
-mod litconv;
+pub mod litconv;
 mod mk_stl;
 mod num;
 mod runtime_error;
 mod str;
 
+pub use assertion_error::AssertionError;
 pub use cpsio::{handle as handleIO, IO};
 pub use mk_stl::mk_stl;
+pub use runtime_error::RuntimeError;
 
 pub use self::bool::Boolean;
 pub use self::num::Numeric;
diff --git a/src/stl/runtime_error.rs b/src/stl/runtime_error.rs
index ad427a5..98a0a3b 100644
--- a/src/stl/runtime_error.rs
+++ b/src/stl/runtime_error.rs
@@ -11,6 +11,8 @@ pub struct RuntimeError {
 }
 
 impl RuntimeError {
+  /// Construct, upcast and wrap in a Result that never succeeds for easy
+  /// short-circuiting
   pub fn fail<T>(
     message: String,
     operation: &'static str,
@@ -18,6 +20,7 @@ impl RuntimeError {
     return Err(Self { message, operation }.into_extern());
   }
 
+  /// Construct and upcast to [ExternError]
   pub fn ext(message: String, operation: &'static str) -> Rc<dyn ExternError> {
     return Self { message, operation }.into_extern();
   }
diff --git a/src/stl/str/char_at.rs b/src/stl/str/char_at.rs
index cc88d53..47d9de0 100644
--- a/src/stl/str/char_at.rs
+++ b/src/stl/str/char_at.rs
@@ -1,38 +1,17 @@
-use std::fmt::Debug;
-
 use super::super::litconv::{with_str, with_uint};
 use super::super::runtime_error::RuntimeError;
-use crate::representations::interpreted::{Clause, ExprInst};
-use crate::representations::{Literal, Primitive};
-use crate::{atomic_impl, atomic_redirect, externfn_impl};
+use crate::interpreted::Clause;
+use crate::{write_fn_step, Literal, Primitive};
 
-/// Takes an uint and a string, finds the char in a string at a 0-based index
-///
-/// Next state: [CharAt1]
-#[derive(Clone)]
-pub struct CharAt2;
-externfn_impl!(CharAt2, |_: &Self, x: ExprInst| Ok(CharAt1 { x }));
-
-/// Prev state: [CharAt2]; Next state: [CharAt0]
-#[derive(Debug, Clone)]
-pub struct CharAt1 {
-  x: ExprInst,
-}
-atomic_redirect!(CharAt1, x);
-atomic_impl!(CharAt1);
-externfn_impl!(CharAt1, |this: &Self, x: ExprInst| {
-  with_str(&this.x, |s| Ok(CharAt0 { s: s.clone(), x }))
-});
-
-/// Prev state: [CharAt1]
-#[derive(Debug, Clone)]
-pub struct CharAt0 {
-  s: String,
-  x: ExprInst,
-}
-atomic_redirect!(CharAt0, x);
-atomic_impl!(CharAt0, |Self { s, x }: &Self, _| {
-  with_uint(x, |i| {
+write_fn_step!(pub CharAt2 > CharAt1);
+write_fn_step!(
+  CharAt1 {}
+  CharAt0 where s = |x| with_str(x, |s| Ok(s.clone()))
+);
+write_fn_step!(
+  CharAt0 { s: String }
+  i = |x| with_uint(x, Ok)
+  => {
     if let Some(c) = s.chars().nth(i as usize) {
       Ok(Clause::P(Primitive::Literal(Literal::Char(c))))
     } else {
@@ -41,5 +20,5 @@ atomic_impl!(CharAt0, |Self { s, x }: &Self, _| {
         "indexing string",
       )?
     }
-  })
-});
+  }
+);