Getting rid of value restriction errors

Question

in OCaml

Objective Caml version 3.11.0

# let rec last l=
    match l with
    [] -> failwith("Empty list")
    |a::[] -> a
    |a::r -> last r;;
val last : 'a list -> 'a = <fun>
# last [];;
Exception: Failure "Empty list".

In F#

>let rec last l = 
    match l with
    [] -> failwith("Empty list")
    | a::[] -> a
    | a::r -> last r;;

val last : 'a list -> 'a

>last [];;
 last [];;
 ^^^^^^^

 stdin(8,1): error FS0030: Restriction de valeur....

>last ([]:int list);;

System.Exception: Empty list
   à FSI_0002.last[a](FSharpList`1 l)
   à <StartupCode$FSI_0003>.$FSI_0003.main@()
Arrêt en raison d'une erreur

What whould I do to be able to pass the empty list as argument without triggering a value restriction error ?

Possibly see also http://stackoverflow.com/questions/1131456/understanding-f-value-restriction-errors — Brian, Jun 15 '11 at 17:32

score 1 · Accepted Answer · answered Jun 15 '11 at 14:13

1

I think you're going to have to put a type annotation somewhere, either on the empty list (as you have) or on the result of the call to last: (last [] : int).

answered Jun 15 '11 at 14:13

kvb

54,864
2
91
133

@Johan - Imagine that your first case returned `Unchecked.defaultof<_>` instead of throwing an exception. Then what would you expect `last []` to give you in the interpreter? – kvb Jun 15 '11 at 14:35
It would have given another type of exception ? Just guessing. – Johan Buret Jun 15 '11 at 14:58
At the time I write this ( 2011-06-16 ) there seems to be no better way. – Johan Buret Jun 16 '11 at 15:18

score 0 · Answer 2 · answered Jun 15 '11 at 14:14

0

You can do

last<obj> []

But fsi will give you a slapped wrist because last never explicitly declare it's type parameter.

answered Jun 15 '11 at 14:14

Massif

4,327
23
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Getting rid of value restriction errors

in OCaml

In F#

2 Answers2