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F3 / F3.02 / branching

F3.02 · part 3 of 3

Branching with case, with & guards

Matching earns its keep when it chooses what runs next. Function heads match arguments to pick a clause; case matches a value against several patterns; with chains matches and stops at the first that fails; and guards add conditions with when.

The portal's sign-in is a sequence of steps that can each fail: verify the magic-link token, load the learner, start a session. That is exactly the shape with was made for — a happy path of successful matches, and one place to handle whatever went wrong.

The sign-in flow The four forms

The sign-in flow

The with below runs three steps, each returning {:ok, _} or {:error, _}. Toggle any step to fail and watch where the pipeline short-circuits — control jumps straight to the else, carrying the error.

with · toggle a step to fail

verify load_user start_session {:ok, session} 

      

    

    
One happy path, read top to bottom, and a single else for every
      failure. No nested conditionals, and no way to forget a branch — an unmatched error falls through
      to else.

  


  

    
The four forms

    

      
The flow above leans on with, but the same matching drives the other branching
        forms. The portal uses each: function heads to handle progress events by tag, case
        to inspect a single result, and guards to add conditions such as an unexpired token.

    

    
# function heads: one clause per event shape
def handle({:lesson_completed, user_id, lesson_id}), do: Progress.mark(user_id, lesson_id)
def handle({:lesson_started, _user_id, _lesson_id}), do: :ok

# case: branch on one value
case Portal.Auth.verify(token) do
  {:ok, claims} -> sign_in(claims)
  {:error, reason} -> deny(reason)
end

# guard: a condition on the match
def valid?(%{exp: exp}) when exp > now(), do: true

    

      

        
F1 · equations by cases

        
A function defined piecewise: a different rule for each shape of input.

      

      
      

        
F3 · matching dispatch

        
Function heads, case, and with choose a branch by matching the value.

      

    

    
That completes F3.02. The portal can now read a magic-link result and act on it. Next module:
      F3.03 — Functions, modules & the pipe. Return to the
      chapter overview for the full path.

  


  

    
References

    
Primary sources for this lesson, and where it connects in the course.

    

      
Sources

      
      
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