X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=using_the_programming_languages.mdwn;h=2097dea239a5069e01e38cba1f749314dd013e18;hp=6a87070b28c34121259cc5c28ac316d3c66c29df;hb=227503eac084d04b374a829310b76ebaec0b2c0e;hpb=7a29e788593675531e78ce0b29858d56fef39e3c
diff --git a/using_the_programming_languages.mdwn b/using_the_programming_languages.mdwn
index 6a87070b..2097dea2 100644
--- a/using_the_programming_languages.mdwn
+++ b/using_the_programming_languages.mdwn
@@ -110,18 +110,35 @@ Jim converted this to OCaml and bundled it with a syntax extension that makes
it easier to write pure untyped lambda expressions in OCaml. You don't have to
know much OCaml yet to use it. Using it looks like this:
- let zero = < z>>
- let succ = < s (n s z)>>
- let one = << $succ$ $zero$ >>
- let two = << $succ$ $one$ >>
- let add = < n $succ$ m>>
+ let zero = << fun s z -> z >>;;
+ let succ = << fun n s z -> s (n s z) >>;;
+ let one = << $succ$ $zero$ >>;;
+ let two = << $succ$ $one$ >>;;
+ let add = << fun m n -> n $succ$ m >>;;
(* or *)
- let add = < fun s z -> m s (n s z)>>
+ let add = << fun m n -> fun s z -> m s (n s z) >>;;
church_to_int << $add$ $one$ $two$ >>;;
- : int = 3
To install Jim's OCaml bundle, DO THIS...
+
+ Some notes:
+
+ * When you're talking to the interactive OCaml program, you have to finish complete statements with a ";;". Sometimes these aren't necessary, but rather than learn the rules yet about when you can get away without them, it's easiest to just use them consistently, like a period at the end of a sentence.
+
+ * What's written betwen the `<<` and `>>` is parsed as an expression in the pure untyped lambda calculus. The stuff outside the angle brackets is regular OCaml syntax. Here you only need to use a very small part of that syntax: `let var = some_value;;` assigns a value to a variable, and `function_foo arg1 arg2` applies the specified function to the specified arguments. `church_to_int` is a function that takes a single argument --- the lambda expression that follows it, `<< $add$ $one$ $two$ >>` -- and, if that expression when fully reduced or "normalized" has the form of a "Church numeral", it converts it into an "int", which is OCaml's (and most language's) primitive way to represent small numbers. The line `- : int = 3` is OCaml telling you that the expression you just had it evaluate simplifies to a value whose type is "int" and which in particular is the int 3.
+
+ * If you call `church_to_int` with a lambda expression that doesn't have the form of a Church numeral, it will complain. If you call it with something that's not even a lambda expression, it will complain in a different way.
+
+ * The `$`s inside the `<<` and `>>` are essentially corner quotes. If we do this: `let a = << x >>;; let b = << a >>;; let c = << $a$ >>;;` then the OCaml variable `b` will have as its value an (atomic) lambda expression, consisting just of the variable `a` in the untyped lambda calculus. On the other hand, the OCaml variable `c` will have as its value a lambda expression consisting just of the variable `x`. That is, here the value of the OCaml variable `a` is spliced into the lambda expression `<< $a$ >>`.
+
+ * The expression that's spliced in is done so as a single syntactic unit. In other words, the lambda expression `<< w x y z >>` is parsed via usual conventions as `<< (((w x) y) z) >>`. Here `<< x y >>` is not any single syntactic constituent. But if you do instead `let a = << x y >>;; let b = << w $a$ z >>`, then what you get *will* have `<< x y >>` as a constituent, and will be parsed as `<< ((w (x y)) z) >>`.
+
+ * `<< fun x y -> something >>` is equivalent to `<< fun x -> fun y -> something >>`, which is parsed as `<< fun x -> (fun y -> (something)) >>` (everything to the right of the arrow as far as possible is considered together). At the moment, this only works for up to five variables, as in `<< fun x1 x2 x3 x4 x5 -> something >>`.
+
+ * The `<< >>` and `$`-quotes aren't part of standard OCaml syntax, they're provided by this add-on bundle. For the most part it doesn't matter if other expressions are placed flush beside the `<<` and `>>`: you can do either `<< fun x -> x >>` or `<x>>`. But the `$`s *must* be separated from the `<<` and `>>` brackets with spaces or `(` `)`s. It's probably easiest to just always surround the `<<` and `>>` with spaces.
+
5. To play around with a **typed lambda calculus**, which we'll look at later