Transform an expression into a fonction : unapply

### 2.15.2 Transform an expression into a fonction : `unapply`

`unapply` is used to transform an expression into a function.
`unapply` takes two arguments an expression and the name of a variable.
`unapply` returns the function defined by this expression and this variable.

Warning when a function is defined, the right member of the affectation is not evaluated, hence `g:=sin(x+1); f(x):=g` does not defined the function f: x → sin(x+1) but defines the function f: x → g. To defined the former function, `unapply` should be used, like in the following example:
Input :

`g:= sin(x+1); f:=unapply(g,x)`

Output :

`(sin(x+1), (x)->sin(x+1))`

hence, the variable `g` is assigned to a symbolic expression and the variable `f` is assigned to a function.
Input :

`unapply(exp(x+2),x)`

Output :

`(x)->exp(x+2)`

Input :

`f:=unapply(lagrange([1,2,3],[4,8,12]),x)`

Output :

`(x)->4+4*(x-1)`

Input :

`f:=unapply(integrate(log(t),t,1,x),x)`

Output :

`(x)->x*log(x)-x+1`

Input :

`f:=unapply(integrate(log(t),t,1,x),x)`
`f(x)`

Output :

`x*log(x)-x+1`

Remark Suppose that f is a function of 2 variables f:(x,w)→ f(x,w), and that g is the function defined by g: w → hw where hw is the function defined by hw(x)=f(x,w).
`unapply` is also used to define g with `Xcas`.
Input :

`f(x,w):=2*x+w`
`g(w):=unapply(f(x,w),x)`
`g(3)`

Output :

`x->2``·``x+3`