Chapter 8  Language extensions

This chapter describes language extensions and convenience features that are implemented in OCaml, but not described in the OCaml reference manual.

19  Extended indexing operators

(Introduced in 4.06)

dot-ext::=  
 ∣ dot-operator-char  { operator-char }  
 
dot-operator-char::= ! ∣   ? ∣  core-operator-char ∣  % ∣  :  
 
expr::= ...  
 ∣ expr .  [module-path .]  dot-ext  ( ( expr ) ∣  [ expr ] ∣  { expr } )  [ <- expr ]  
 
operator-name::= ...  
 ∣ . dot-ext  (() ∣  [] ∣  {}) [<-]  
 

This extension provides syntactic sugar for getting and setting elements for user-defined indexed types. For instance, we can define python-like dictionaries with

module Dict = struct include Hashtbl let ( .%{} ) tabl index = find tabl index let ( .%{}<- ) tabl index value = add tabl index value end let dict = let dict = Dict.create 10 in let () = dict.Dict.%{"one"} <- 1; let open Dict in dict.%{"two"} <- 2 in dict
dict.Dict.%{"one"};;
- : int = 1
let open Dict in dict.%{"two"};;
- : int = 2

19.1  Multi-index notation

expr::= ...  
 ∣ expr .  [module-path .]  dot-ext (  expr  {; expr }+ )  [ <- expr ]  
 ∣ expr .  [module-path .]  dot-ext [  expr  {; expr }+ ]  [ <- expr ]  
 ∣ expr .  [module-path .]  dot-ext {  expr  {; expr }+ }  [ <- expr ]  
 
operator-name::= ...  
 ∣ . dot-ext  ((;..) ∣  [;..] ∣  {;..}) [<-]  
 

Multi-index are also supported through a second variant of indexing operators

let (.%[;..]) = Bigarray.Genarray.get let (.%{;..}) = Bigarray.Genarray.get let (.%(;..)) = Bigarray.Genarray.get

which is called when an index literals contain a semicolon separated list of expressions with two and more elements:

let sum x y = x.%[1;2;3] + y.%[1;2] (* is equivalent to *) let sum x y = (.%[;..]) x [|1;2;3|] + (.%[;..]) y [|1;2|]

In particular this multi-index notation makes it possible to uniformly handle indexing Genarray and other implementations of multidimensional arrays.

module A = Bigarray.Genarray let (.%{;..}) = A.get let (.%{;..}<- ) = A.set let (.%{ }) a k = A.get a [|k|] let (.%{ }<-) a k x = A.set a [|k|] x let syntax_compare vec mat t3 t4 = vec.%{0} = A.get vec [|0|] && mat.%{0;0} = A.get mat [|0;0|] && t3.%{0;0;0} = A.get t3 [|0;0;0|] && t4.%{0;0;0;0} = t4.{0,0,0,0}
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