OCaml: Call function within another function

问题

In one module I have a user defined type and a recursive function that returns a string. Then I want to create a function that will create an object of that type and pass it to the function. Here is a simple example of the code I have:

type species =
    Animal of string
    | Mammal of species
    | Fish of species

let rec species_to_string = function
    | Animal (x) -> x
    | Mammal (x) -> "Mammal (" ^ (species_to_string x) ^ ")"
    | Fish (x) -> "Fish (" ^ (species_to_string x) ^ ")"

let process () =    
    let dog = Mammal(Animal("Dog"))
    let dogstring = species_to_string dog
    print_string dogstring

However when I try to compile this, I receive the error:

File "test.ml", line 13, characters 1-4:
Error: Syntax error

where line 13 is the second last line in my example above.

My code doesn't seem to be the issue. When I change the code to this:

type species =
    Animal of string
    | Mammal of species
    | Fish of species;;

let rec species_to_string = function
    | Animal (x) -> x
    | Mammal (x) -> "Mammal (" ^ (species_to_string x) ^ ")"
    | Fish (x) -> "Fish (" ^ (species_to_string x) ^ ")";;

let dog = Mammal(Animal("Dog"));;
let dogstring = species_to_string dog;;
print_string dogstring;;

it compiles and runs correctly. But I need to put the last 3 lines in a function so it can be called by another module. What am I doing wrong?

回答1:

You need to say

let dog = Mammal(Animal("Dog")) in
let dogstring = species_to_string dog in
print_string dogstring

That is, you need to use the keyword in.

Longer explanation: there are two different uses of let in OCaml. At the top level of a module it defines the contents of the module. This is the case for your definitions of species_to_string and process. In these cases it appears without in.

In all other cases (inside the outermost definitions) the only allowed form is let var = expr in expr. I.e., the in keyword is required.

Having two different uses for let is confusing, there's no question. But once you get used to it it's OK.

来源：https://stackoverflow.com/questions/36414762/ocaml-call-function-within-another-function

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ocaml