CODEWITHSUNDEEP
ocaml
Addem
Addem

Reputation: 3949

How to partially match a pattern in OCaml

I have a list lst of objects of type value where

type value = A of int | B of bool | C of string

In doing some matching on the the list, I tried to write 

match lst with 
| A x :: val :: tl -> ...

and got an exception saying that in the variable val a pattern was expected. I am assuming this is because in the head of the list I matched on a value variant, but for val I wanted to capture all possible next entries in the list. I can think of some ways around them, like writing several cases for the several variants of val. But since I want to do the same basic thing no matter what val is, that seems like a very inelegant solution. Is there a better solution?

Upvotes: 0

Views: 294

Answers (1)

Shon
Shon

Reputation: 4098

Elaborating an answer based on glennsl's comment, I assume this snippet entered into the top level is reproducing the syntax error you're hitting:

image of the error at the top level

since val is a reserved keyword, it is not legal to use it in pattern matches. The error is saying that the underlined token val is triggering a syntax error since it is expecting something that could be part of a pattern.

The following should compile without any problems (using some random values for example):

type value = A of int | B of bool | C of string

match [A 1; B true; C "foo"] with
| A x :: v :: tl -> Some (x, v)
| _ -> None

And this is simply due to the replacement of the keyword val with the variable v in the pattern.

Upvotes: 3

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