Transitional and Strict modes

Transitional and Strict modes

Since version 5.00, Camlp5 has been able to be installed in two modes: the transitional mode and the strict mode. When Camlp5 is installed, it works with one only of these modes (the two modes contain indeed different definitions of some interfaces and are incompatible with one another). The user must choose in which mode he wants to use Camlp5.

This notion has been introduced to ensure backward compatibility of the Camlp5 syntax tree, together with the usage of a new quotation kit “q_ast.cmo”, which allows to use Camlp5 syntax tree quotations in user syntax (with all its possible extensions).

A short example of these syntax tree quotations:

If the syntax of the extensible grammars has been added, it is possible to write things like:

<:expr< EXTEND a: [ [ c = d -> $e$ ] ]; END >>;

representing the syntax tree of this statement: this is not possible with the classical quotation kit “q_MLast.cmo” because all quotations must be there only in revised syntax and without syntax extensions.

Here are the differences between the two modes:

Transitional

Compatibility

The syntax tree is fully compatible with the previous versions of Camlp5, no changes has to be done in the users’ programs.

Quotation kit “q_ast.cmo”

The antiquotations are not available: when used, a syntax error message is displayed.

Strict

Compatibility

The syntax tree is different, users’ programs may have to be modified, but not necessarily.

Quotation kit “q_ast.cmo”

All antiquotations are available.

In strict mode, the programs have more chances to be compatible with the previous versions if they use syntax tree quotations rather than syntax tree nodes. A solution is therefore to change the expressions and patterns using nodes into expressions and patterns using quotations (which is backward compatible).

Which mode is installed ?

To determine the mode of an installed version of Camlp5, type:

camlp5 -pmode

Selecting mode when compiling Camlp5

When compiling Camlp5 from source, the mode must first be selected at configuration time. The configure script must be run with one of these options:

./configure -strict
./configure -transitional

The default is “transitional”, i.e. without option, the sources are compiled in transitional mode.