The array syntax of Fortran is extended with additional trailing subscripts in square brackets to provide a concise representation of references to data that is spread across images.
Another implementation of coarrays and related parallel extensions from Fortran 2008 is available in the OpenUH compiler (a branch of Open64) developed at the University of Houston.
Some implementations, such as the ones available in the GNU Fortran and OpenUH compilers, may run on top of other low-level layers (for example, GASNet) designed for supporting partitioned global address space languages.
These enable the user to write a more efficient version of the above algorithm where the lack of explicit synchronization offers the potential for higher performance due to less coordination between the images.
[3] Their perspective is that the Fortran 2008 standard committee's design choices were shaped more by the desire to introduce as few modifications to the language as possible than to assemble the best set of extensions to support parallel programming.