Conisder adding ensure(val, pat, how:Fun)

[ilyash-b]

[ilyash-b]

Experimental implementation is in the dev branch.

doc EXPERIMENTAL! Do not use!
doc Ensures that value matches pattern.
doc If the value matches, it is returned as is.
doc If not, Value(how, x) is used instead.
doc In this case, there is assert() to see that the new value is matching pat.
doc %EX - ensure(param, Str, encode_json)
doc %STATUS - experimental
F ensure(x, pat, how) {
	x.when(Not(pat), {
		how.Value(x)::{
			assert(A, pat, Error('ensure() failed to transform to desired pattern').set(x=x, pat=pat, how=how))
		}
	})
}