# A.2.11 Definitions

Although the rules we listed so far allows us to construct everything we need directly, we would still like to be able to use named constants, such as $\mathsf{isequiv}$, as a matter of convenience. Informally, we can think of these constants simply as abbreviations, but the situation is a bit subtler in the formalization.

For example, consider function composition, which takes $f:A\to B$ and $g:B\to C$ to $g\circ f:A\to C$. Somewhat unexpectedly, to make this work formally, $\circ$ must take as arguments not only $f$ and $g$, but also their types $A$, $B$, $C$:

 ${\circ}:\!\!\equiv{\lambda}(A\,{:}\,\mathcal{U}_{i}).\,{\lambda}(B\,{:}\,% \mathcal{U}_{i}).\,{\lambda}(C\,{:}\,\mathcal{U}_{i}).\,{\lambda}(g\,{:}\,B\to C% ).\,{\lambda}(f\,{:}\,A\to B).\,{\lambda}(x\,{:}\,A).\,g(f(x)).$

From a practical perspective, we do not want to annotate each application of $\circ$ with $A$, $B$ and $C$, as the are usually quite easily guessed from surrounding information. We would like to simply write $g\circ f$. Then, strictly speaking, $g\circ f$ is not an abbreviation for ${\lambda}(x\,{:}\,A).\,g(f(x))$, because it involves additional implicit arguments which we want to suppress.

Inference of implicit arguments, typical ambiguity (§1.3 (http://planetmath.org/13universesandfamilies)), ensuring that symbols are only defined once, etc., are collectively called elaboration. Elaboration must take place prior to checking a derivation, and is thus not usually presented as part of the core type theory. However, it is essentially impossible to use any implementation of type theory which does not perform elaboration; see [Coq, norell2007towards] for further discussion.

Title A.2.11 Definitions
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