Where does fold collide?

A fold is a bend or flexure in layers of rock, formed when originally flat strata are squeezed or bent by pressure in the Earth's crust. Sir James Hall's 1815 study On the Vertical Position and Convolution of Certain Strata gave one of the first accounts of such bending, reproducing folded layers by compressing sheets of cloth, and John Ramsay's 1967 textbook Folding and Fracturing of Rocks set out the modern geometric description of a fold's shape, axis, and limbs. Here the referent is a physical curve frozen into solid rock, mapped by its angle and orientation in the field, and it has nothing to do with a protein taking its shape, a slice of a dataset, or a function that collapses a list.[1][2]

Unit

A single bend in a rock layer, described by its axis, limbs, and angle

Recovery means

Not applicable

Measured by

Mapped in the field and from cross-sections by strike, dip, and fold geometry

Protein folding is the process by which a chain of amino acids settles into the specific three-dimensional shape that lets it work. Anfinsen, Haber, Sela, and White's 1961 experiment The Kinetics of Formation of Native Ribonuclease showed that an unravelled protein could spontaneously return to its correct working shape on its own, and Anfinsen's 1973 review Principles That Govern the Folding of Protein Chains drew the general rule that a protein's amino-acid order alone can determine its folded form. Here the referent is a molecule arranging itself in space, measured by its structure and stability, and it is unrelated to a bend in rock, a slice of a dataset, or an operation on a list.[3][4]

Unit

One polypeptide chain reaching its native three-dimensional structure

Recovery means

A denatured chain can often refold to its native form when normal conditions return

Measured by

Resolved by X-ray crystallography, NMR, and stability assays of the folded state

In functional programming, a fold is a higher-order function that walks through a data structure such as a list and combines all its elements into a single result using a given step operation, for example adding up every number in a list. Bird and Wadler's 1988 text Introduction to Functional Programming named and set out the fold operators over lists, and Meijer, Fokkinga, and Paterson's 1991 paper Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire generalised the pattern to any recursive structure as the catamorphism. Here the referent is an operation in a program that collapses a structure into one value, defined by the step it applies and its starting value, and it has no connection to a bend in rock, a protein's shape, or a slice of a dataset.[5][6]

Unit

One application of a fold, taking a structure and a step operation to one result

Recovery means

Not applicable

Measured by

Characterised by its step function, starting value, and left or right association