A function value doesn't have any structure---at least none that's visible to the pattern-matching system. You can only match against simple patterns like `_` or the variable `f`.
-When matching a variable against a λ-generated function value in a `let`- or `letrec`-construction, there's an alternative syntax that you may find more convenient. This:
+When matching a λ-generated function value against a variable in a `let`- or `letrec`-construction, there's an alternative syntax that you may find more convenient. This:
`let`
`f match` λ`x.` φ`;`