How to fix the code errors in OCaml? - ocaml

Question

I'm new to OCaml,and have just found an on-line OCaml guide, http://try.ocamlpro.com/.
When it comes to lesson 5, I get trapped in some exercises. So, here is the problem:
fix all these let expressions in order to get the expected result at the end:
1.
let xy =
let x = 'x' and let y = 'y' in x ::[y]
2.
let ab =
let a = 'a'
let b = 'B' in Char.lowercase b
in a ::[b]
3.
let up = Char.uppercase in
big_xy = List.map up xy ;
big_ab = List.map up ab ;
big_ab # big_xy
PS:I know how to solve problem 1,and I post it here for the need of problem 3.For Problem 2 and 3,I can see where the problem is,but I can hardly figure out how to fix it.I'm a little confused about the compact "let...in" statement,and there is few resource I can refer to for such problem.

Answer 1

This is a completely wrong OCaml, indeed it is not an OCaml at all))
I'm just trying to guess what was your intention, so in the second code sample you have tried to bind to variables to some character values, then lowercase one of them and create a list of this two values.
Before you can understand, what is wrong with your code, you need to know something about OCaml's grammar.
There're several kinds of binding statements in OCaml, the first one is an expression and has the following pattern:
let «variable-name» = «expr-1» in «expr-2»
This means that you bind the result of evaluating «expr-1» to a «variable-name», so that you can use it inside «expr-2» (and only in «expr-2», it is its scope). «expr-2» can be another binding-expression on its own. Let's take some examples:
let a = 'a' in [a]
or
let a = 'a' in
let b = 'b' in
[a; b]
The second kind of bindings is a so called top-level binding, that is used when you're adding some definition to a module level (or an interactive top-level, that is the same). They look a little bit simplier, but they're not expressions. When I say that something is not an expression, I mean, mostly, that it can't be evaluated to a value. So, they have the following pattern:
let «variable-name» = «expr»
As you can see, there is no in part, only raw binding. You can use this statement to create a global constant, e.g.,
let pi = 4. *. atan 1.
Or, you can use it to bind a function to a name (in other words to write a function definition):
let circle_area radius = pi * radius ** 2.
Of course, inside a function you can use any expressions, including a binding expression:
let area_of_circles radii =
let areas = List.map circle_area radii in
List.fold_left (+.) 0. areas
I would like to note, that there're lots of good tutorials and textbooks, that can help you to learn OCaml very fast. You can find them at the OCaml tag page. I, personally, would suggest you to look at the 'OCaml from the very beginning' book.