Control flow

L4 has no statements, only expressions, so choosing between results is done with expressions that give a value. IF and BRANCH choose by conditions; CONSIDER chooses by the shape of a value. In each, every outcome must have the same type.

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• IF: IF … THEN … ELSE, one condition and two outcomes
• BRANCH: several conditions, tried from top to bottom, with OTHERWISE
• CONSIDER: pattern matching with WHEN … THEN and OTHERWISE, and the check for missing branches

A function can also be written as several clauses, one per case, which means the same as a CONSIDER. Conditions are combined with AND, OR, NOT, IMPLIES and UNLESS.

The Foundation course, module 3 teaches these step by step.