Writing checkpoint

projects/Idris/src/LessMacrosMoreTypes/Printf.md

@@ -3,7 +3,8 @@
 ```idris hide
 module LessMacrosMoreTypes.Printf
 
-%default total
+import Data.List
+import System
 ```
 
 While C can provide convenient string formatting by having hideously memory
@@ -33,10 +34,19 @@ equivalent:
 
 ```idris
 failing
-  example_usage : String
-  example_usage = printf "%s %d %2d" "hello" 1 2
+  printf "%s %d %2d" "hello" 1 2
 ```
 
+Idris lacks a dedicated facility for variadics, but we can call functions in
+type signatures, Idris allows us to manipulate types as first class values, and
+we can use the runtime values of previous arguments to the function we are
+defining as arguments to our type-level function.
+
+To get our variadic `printf` function, we can parse our format string into a
+data structure, then pass that data structure into a type-level function that
+calculates the rest of the type signature of our `printf` function based on its
+contents.
+
 ## Parsing a Format String
 
 First, we need a data structure to describe our format string. We define the
@@ -49,7 +59,7 @@ data Format : Type where
   Number : (next : Format) -> Format
   ||| A slot that should be filled  in with a number, padded to a certian number
   ||| of digits
-  PaddedNumber : (digits : Nat) -> Format
+  PaddedNumber : (digits : Nat) -> (next : Format) -> Format
   ||| A slot that should be filled in with a string
   Str : (next : Format) -> Format
   ||| A literal component of the format string that should not be interpolated
@@ -58,8 +68,168 @@ data Format : Type where
   End : Format
 ```
 
+We'll need to be able to parse numbers for our `PaddedNumber` constructor, so
+we'll write a little helper function to handle that component of the parsing.
+Simply keep pulling off characters, converting them to integers by shifting
+their ordinal values.
+
+> [!WARNING]
+> On Idris 2's chez scheme backend, these are unicode ordinals, where the digit
+> characters are in numerical order, but the values are backend dependent and
+> this code is not guaranteed to work properly on other backends.
+
+```idris
+parseNumber : (xs : List Char) -> (acc : Nat) -> (Nat, List Char)
+parseNumber [] acc = (acc, [])
+parseNumber (x :: xs) acc = 
+  if isDigit x
+    then let value = cast $ (ord x) - (ord '0')
+    in parseNumber xs (acc * 10 + value)
+    else (acc, x :: xs)
+```
+
+We'll also want another one to scoop up a literal into a `List Char`, continuing
+until we hit the end of the string or the next `%`. This is optional, we could
+have each literal consist of an individual char, but we will go ahead and group
+them together.
+
+```idris
+parseLiteral : (xs : List Char) -> (List Char, List Char)
+parseLiteral [] = ([], [])
+parseLiteral ('%' :: xs) = ([], '%' :: xs)
+parseLiteral (x :: xs) = 
+  let (literal, rest) = parseLiteral xs
+  in (x :: literal, rest)
+```
+
+Parse our format string into our `Format` data structure. The specifics of the
+parsing here aren't really material to the main point of this article, but we
+use a basic pattern matching approach, calling into our helper functions as
+appropriate. 
+
+```idris
+parseFormat : (xs : List Char) -> Maybe Format
+parseFormat [] = Just End
+-- A `%` has to come before a specifier
+parseFormat ('%' :: []) = Nothing
+parseFormat ('%' :: (x :: xs)) = 
+  if isDigit x
+    -- Invoke parseNumber to get our padding specifier
+    then let (digits, rest) = parseNumber (x :: xs) 0
+    in case rest of
+      -- A padding specifier has to come before something
+      [] => Nothing
+      ('d' :: ys) => do
+        rest <- parseFormat ys
+        Just $ PaddedNumber digits rest
+      -- A padding specifier is only valid before a number specifier
+      (y :: ys) => Nothing
+    -- Parse as an unpadded specifier
+    else case x of
+      'd' => do
+        rest <- parseFormat xs
+        Just $ Number rest
+      's' => do
+        rest <- parseFormat xs
+        Just $ Str rest
+      -- Any other character here is an invalid specifier
+      _ => Nothing
+parseFormat (x :: xs) = 
+  let (literal, rest) = parseLiteral (x :: xs)
+  in do
+    rest <- parseFormat rest
+    Just $ Literal (pack literal) rest
+```
+
 ## Calculating a Type From a Format String
 
+```idris 
+PrintfType' : Format -> Type
+PrintfType' (Number next) =
+  (num : Nat) -> PrintfType' next
+PrintfType' (PaddedNumber digits next) =
+  (num : Nat) -> PrintfType' next
+PrintfType' (Str next) = 
+  (str : String) -> PrintfType' next
+PrintfType' (Literal literal next) = 
+  PrintfType' next
+PrintfType' End = String
+
+PrintfType : Maybe Format -> Type
+PrintfType Nothing = Void -> String
+PrintfType (Just x) = PrintfType' x
+```
+
 ## printf
 
-## Working with run-time format strings
+### With the Format Structure
+
+```idris hide
+left_pad : (len : Nat) -> (pad : Char) -> (str : String) -> String
+left_pad len pad str = 
+  let cs = unpack str
+  in if length cs < len
+    then pack $ replicate (len `minus` length cs) pad ++ cs
+    else str
+```
+
+```idris
+printfFmt : (fmt : Maybe Format) -> (acc : String) -> PrintfType fmt
+printfFmt Nothing acc = 
+  \void => absurd void
+printfFmt (Just x) acc = printfFmt' x acc
+  where 
+    printfFmt' : (fmt : Format) -> (acc : String) -> PrintfType' fmt
+    printfFmt' (Number next) acc = 
+      \i => printfFmt' next (acc ++ show i)
+    printfFmt' (PaddedNumber digits next) acc = 
+      \i => printfFmt' next (acc ++ left_pad digits '0' (show i))
+    printfFmt' (Str next) acc =
+      \str => printfFmt' next (acc ++ str)
+    printfFmt' (Literal literal next) acc = 
+      printfFmt' next (acc ++ literal)
+    printfFmt' End acc = acc
+```
+
+### With a Format String
+
+```idris
+printf : (fmt : String) -> PrintfType (parseFormat (unpack fmt))
+printf fmt = printfFmt _ ""
+```
+
+We can call `printf` as expected, with the number of and types of the arguments
+being determined by the provided format string:
+
+```idris hide
+-- @@test printf hello world
+helloWorld : IO Bool
+helloWorld = do
+  pure $
+```
+
+```idris
+  printf "%s %s%s %3d %d" "Hello" "world" "!" 1 23 == "Hello world! 001 23"
+```
+
+It will even fail to compile if you attempt to provide arguments to an invalid
+format string, which we can demonstrate by trying to apply a padding modifier to
+a string specifier:
+
+> [!NOTE]
+> `failing` blocks have an additional feature, they will trigger a compiler
+> error if their contents _do_ compile successfully.
+
+```idris
+failing
+  printf "Hello %s %3s" "world" "!"
+```
+
+## Working With Run-Time Format Strings
+
+## Conclusions
+
+### Why this API isn't really a great idea
+
+<!-- Doesn't fail to compile if you don't pass any arguments --> 
+