In functional languages intermediate data structures are often used as glue to connect separate parts of a program together. These intermediate data structures are useful because they allow modularity, but they are also a cause of inefficiency: each element need to be allocated, to be examined, and to be deallocated. Warm fusion is a program transformation technique which aims to eliminate intermediate data structures. Functions in a program are first transformed into the so called build-cata form, then fused via a one-step rewrite rule, the cata-build rule. In the process of the transformation to build-cata form we attempt to replace explicit recursion with a fixed pattern of recursion (catamorphism). We analyse in detail the problem of removing - possibly mutually recursive sets of - polynomial datatypes. Wehave implemented the warm fusion method in the Glasgow Haskell Compiler, which has allowed practical feedback. One important conclusion is that catamorphisms and fusion in general deserve a more prominent role in the compilation process. We give a detailed measurement of our implementation on a suite of real application programs.