Chapter 12 Language extensions

1 Recursive definitions of values

(Introduced in Objective Caml 1.00)

As mentioned in section 11.7.2, the let rec binding construct, in addition to the definition of recursive functions, also supports a certain class of recursive definitions of non-functional values, such as

let rec name₁ = 1 :: name₂ and name₂ = 2 :: name₁ in expr

which binds name₁ to the cyclic list 1::2::1::2::…, and name₂ to the cyclic list 2::1::2::1::…Informally, the class of accepted definitions consists of those definitions where the defined names occur only inside function bodies or as argument to a data constructor.

More precisely, consider the expression:

let rec name₁ = expr₁ and … and name_n = expr_n in expr

It will be accepted if each one of expr₁ … expr_n is statically constructive with respect to name₁ … name_n, is not immediately linked to any of name₁ … name_n, and is not an array constructor whose arguments have abstract type.

An expression e is said to be statically constructive with respect to the variables name₁ … name_n if at least one of the following conditions is true:

e has no free occurrence of any of name₁ … name_n
e is a variable
e has the form fun … -> …
e has the form function … -> …
e has the form lazy ( … )
e has one of the following forms, where each one of expr₁ … expr_m is statically constructive with respect to name₁ … name_n, and expr₀ is statically constructive with respect to name₁ … name_n, xname₁ … xname_m:
- let [rec] xname₁ = expr₁ and … and xname_m = expr_m in expr₀
- let module … in expr₁
- constr (expr₁, … , expr_m)
- `tag-name (expr₁, … , expr_m)
- [| expr₁; … ; expr_m |]
- { field₁ = expr₁; … ; field_m = expr_m }
- { expr₁ with field₂ = expr₂; … ; field_m = expr_m } where expr₁ is not immediately linked to name₁ … name_n
- ( expr₁, … , expr_m )
- expr₁; … ; expr_m

An expression e is said to be immediately linked to the variable name in the following cases:

e is name
e has the form expr₁; … ; expr_m where expr_m is immediately linked to name
e has the form let [rec] xname₁ = expr₁ and … and xname_m = expr_m in expr₀ where expr₀ is immediately linked to name or to one of the xname_i such that expr_i is immediately linked to name.