如何在Ocaml中正确使用分隔符

Question

Consider the following code in OCamel:

let rodrigue="The cid" ; let diegue="cid's father"

this gives a syntax error on tryOcaml. What is wrong? It seems to work when I removed the ; Please help.

Also, rodrigue.[4] <- 'c' gives unknown constructor error. Does this mean that we can't use <- ? as rodrigue.[4] is immutable or there is a function to change a character in a string?

What does ; do ? I tried 4=4 ;6=5 , it gave a value of bool= false.

is 4=4 an expression or a statement?

what about let x=5 ?

So let x=8 let y=9 executes both statements but $4=4 ; 5=6$ only evaluate the last expression?

What is the difference between an expression and a statement?

What is toplevel? What is the difference between toplevel and normal OCamel expression?

Answer 1

There are two places where you can use let in OCaml. At the top level of a module you can have this:

let pattern = expression

A module is, in essence, a collection of named values, and this form of let is the way of defining a name and its value.

On the other hand, there is an OCaml expression that looks like this:

let pattern = expression1 in expression2

This is, in essence, a way of defining local variables for use in expression2 .

Your code with the ; doesn't work as either of these forms. A ; is used to combine two expressions into a single expression. But let pattern = value is not an expression. It has the form of a top-level declaration of a module.

The reason it works when you remove the ; is that you then have two top-level declarations. That will work as long as the code appears at the top level of a module. In this case, it defines the names rodrigue and diegue as named values of the module.

Other answers:

In recent versions of OCaml, strings are not mutable. You need to use the bytes type if you need mutability. As part of this change the string.[...] <- ... notation became obsolete. You should use Bytes.set instead.

# let mystring = "abc";;
val mystring : string = "abc"
# let mybytes = Bytes.of_string "abc";;
val mybytes : bytes = Bytes.of_string "abc"
# mystring.[0] <- 'b';;
Warning 3: deprecated: String.set
Use Bytes.set instead.
Error: This expression has type string but an expression was 
            expected of type bytes
# Bytes.set mybytes 0 'b';;
- : unit = ()
# mybytes;;
- : bytes = Bytes.of_string "bbc"

OCaml doesn't really have statements. In places where you would have a statement in some other languages, you have an expression of type unit . For example, the for statement in OCaml is actually an expression with unit type.

# for x = 1 to 1 do () done = ();;
- : bool = true

From this point of view, then, 4 = 4 is an expression because it has type bool (and not unit ).

So then let x = 5 is neither an expression nor a statement (which is just a kind of expression). As I pointed out above, this has the form of a top-level module declaration. Hence, it can only appear at the top of a module.

In contrast, let x = 5 in x + 2 is an expression with the value 7.

The name toplevel is what OCaml veterans call the read-eval-print loop of OCaml. In other words, it's a name for the OCaml interpreter.

In my answers above I'm using "top level" to mean the outermost syntactic nesting level of a module. It's not the same as the toplevel (the OCaml interpreter).