I am trying to use functional programming in place of imperative programming. One of the things I would like to do is iteration. As an example, to try to convert the following loop

x0 = [1,2];

for i = 1:4
y1 = [x0(1)*x0(2),x0(2)/x0(1)];
y2 = [y1(1)/y2(2),5];
y3 = [(y2(1)+y2(2))^2, 1 + y2(1)];
y4 = [y3(1)/4, y3(2)/6];
x0 = y4;

The analogous functional approach is

step4 = Function[{#[[1]]/4, #[[2]]/6}];
step3 = Function[{(#[[1]] + #[[2]])^2, 1 + #[[1]]}];
step2 = Function[{#[[1]]/#[[2]], 5}];
step1 = Function[{#[[1]]*#[[2]], #[[2]]/#[[1]]}];
total = Composition[step4, step3, step2, step1];
(* alternatively, Function[step4[step3[step2[step1[#]]]]] *)
NestList[total, {2., 3.}, 4]

For the purpose of debugging, I would like to use ComposeList to know, giving an initial starting value, what the value at a given step would be. However, while Composition can be used to generate a pure function, ComposeList must be given a formal parameter. For the use above, with functions defined with list inputs, I have to put in a list of formal parameters like {x,y}, which prevents subsequent use with functions like NestList. For example,

all = ComposeList[{step1, step2, step3, step4}, {x, y}]
uptostep2 = all[[2]];

Is there a way to create pure functions using ComposeList or another method to achieve the same thing?


What about FoldList[Composition, {step1, step2, step3, step4}]?


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