work on slides

slides/code-examples/example.hs

@@ -0,0 +1,6 @@
+hd :: [a] -> a
+hd [] = error "empty list"
+hd (x : _) = x
+
+main :: IO ()
+main = print (hd []::[String])

slides/code-examples/examples.agda-lib

@@ -0,0 +1,3 @@
+name: examples
+include: .
+depend: standard-library

slides/code-examples/reverse.agda

@@ -0,0 +1,40 @@
+{-# OPTIONS --guardedness #-}
+open import Codata.Musical.Colist hiding (_++_)
+open import Codata.Musical.Colist.Bisimilarity
+open import Codata.Musical.Notation
+open import Data.Nat
+open import Data.Nat.Show renaming (show to showℕ)
+open import Function.Base
+open import Relation.Binary.PropositionalEquality
+open import Data.String using (String; _++_)
+
+module reverse where
+  data Delay (A : Set) : Set where
+    now : A → Delay A
+    later : ∞ (Delay A) → Delay A
+
+  foldl : ∀ {A B : Set} → (A → B → A) → A → Colist B → Delay A
+  foldl c n [] = now n
+  foldl c n (x ∷ xs) = later (♯ foldl c (c n x) (♭ xs))
+
+  -- reversing possibly infinite lists
+  reverse : ∀ {A : Set} → Colist A → Delay (Colist A)
+  reverse {A} = reverseAcc []
+    where
+      reverseAcc : Colist A → Colist A → Delay (Colist A)
+      reverseAcc = foldl (λ xs x → x ∷ (♯ xs)) -- 'flip _∷_' with extra steps
+
+  run_for_steps : ∀ {A : Set} → Delay A → ℕ → Delay A
+  run now   x for n     steps = now x
+  run later x for zero  steps = later x
+  run later x for suc n steps = run ♭ x for n steps
+
+
+  fin-colist : Colist ℕ
+  fin-colist = 1 ∷ ♯ (2 ∷ ♯ (3 ∷ ♯ []))
+
+  inf-colist : Colist ℕ
+  inf-colist = 1 ∷ ♯ inf-colist
+
+  -- run reverse fin-colist for 5 steps
+  -- run reverse inf-colist for 1000 steps

slides/main.tex

@@ -137,6 +137,9 @@ Leon Vatthauer%\inst{1}
 % ================================================
 % The main document
 % ------------------------------------------------
+\usepackage{MnSymbol} % for \squaredots
+\usepackage{listings}
+\lstset{mathescape}
 \begin{document}
 % Title page
 \begin{frame}[t,titleimage]{-}

slides/sections/00_intro.tex

@@ -1,3 +1,80 @@
-\begin{frame}[t]{Introduction}{}
+\begin{frame}[t, fragile]{Partiality in Haskell}{}
-An introduction
+Haskell allows users to define arbitrary partial functions, some can be spotted easily by their definition:
+\begin{lstlisting}[language=haskell]
+head :: [a] -> a
+head [] = error "empty list"
+head (x:xs) = x
+\end{lstlisting}
+
+% TODO right of this add error bubble that shows what happens for `head []`
+
+others might be more subtle:
+
+\begin{lstlisting}[language=haskell]
+reverse l = rev l []
+where
+  rev []     a = a
+  rev (x:xs) a = rev xs (x:a)
+\end{lstlisting}
+% TODO right of this add error bubble that shows `reverse ones`
+\end{frame}
+
+\begin{frame}[t, fragile]{Partiality in Agda}{The Maybe Monad}
+In Agda every function has to be total and terminating, so how do we model partial functions?
+
+\begin{lstlisting}
+data Maybe (A : Set) : Set where
+  just : A $\rightarrow$ Maybe A
+  nothing : Maybe A
+\end{lstlisting}
+
+for head we can then do:
+
+\begin{lstlisting}
+head : $\forall$ A $\rightarrow$ List A $\rightarrow$ Maybe A
+head nil = nothing
+head (cons x xs) = just x
+\end{lstlisting}
+
+But what about \lstinline|reverse|?
+\end{frame}
+
+\begin{frame}[t, fragile]{Partiality in Agda}{Capretta's Delay Monad}
+Capretta's Delay Monad is a coinductive data type whose inhabitants can be viewed as suspended computations. 
+\begin{lstlisting}
+data Delay (A : Set) : Set where
+  now : A $\rightarrow$ Delay A
+  later : $\infty$ (Delay A) $\rightarrow$ Delay A
+\end{lstlisting}
+\lstinline|now| lifts a computation, while \lstinline|later| delays it by one time unit.
+
+The delay datatype contains a constant for non-termination:
+\begin{lstlisting}
+never : Delay A
+never = later ($\sharp$ never)
+\end{lstlisting}
+
+and we can define a function for \textit{running} a computation (for some amount of steps):
+
+\begin{lstlisting}
+run_for_steps : Delay A $\rightarrow$ $\mathbb{N}$ $\rightarrow$ Delay A
+run now   x for n     steps = now x
+run later x for zero  steps = later x
+run later x for suc n steps = run $\flat$ x for n steps
+
+\end{lstlisting}
+\end{frame}
+
+\begin{frame}[t, fragile]{Partiality in Agda}{Reversing (possibly infinite) lists}
+\begin{lstlisting}
+foldl : $\forall$ {A B : Set} $\rightarrow$ (A $\rightarrow$ B $\rightarrow$ A) $\rightarrow$ A $\rightarrow$ Colist B $\rightarrow$ Delay A
+foldl c n [] = now n
+foldl c n (x $\squaredots$ xs) = later ($\sharp$ foldl c (c n x) ($\flat$ xs))
+
+reverse : $\forall$ {A : Set} $\rightarrow$ Colist A $\rightarrow$ Delay (Colist A)
+reverse {A} = reverseAcc []
+  where
+    reverseAcc : Colist A $\rightarrow$ Colist A $\rightarrow$ Delay (Colist A)
+    reverseAcc = foldl ($\lambda$ xs x $\rightarrow$ x $\squaredots$ ($\sharp$ xs)) -- 'flip _$\squaredots$_' with extra steps
+\end{lstlisting}
 \end{frame}