hy — consolidated language guide — DedSec Project

Hy

Working model

Hy uses Lisp-style expressions on Python's object model. Macros transform syntax before normal evaluation; host Python objects and imports remain available.

Focused source example

(setv readings [2 4 6])
(print (sum readings))

Create a second list containing zero and a negative number, then show whether the sum changes predictably. Contrast quoting a form with evaluating it.

Evidence to collect

Record the exact Hy compiler or runtime version, required dependencies, and output from harmless test data. Predict an observation before checking it, and keep both the initial and revised source to show which change caused the result. Do not claim that a displayed fragment was executed if the platform is unavailable.

Failure mode worth investigating

Macro expansion and runtime function calls occur at different stages; do not treat Hy syntax as literal Python syntax.

Follow-up challenge

Transform this example using a second input and explain one limiting case that alters its behavior. State where platform-specific libraries or tooling can affect the outcome; if no compatible runtime is installed, document the static reasoning honestly and distinguish it from a passing execution test.

Hy: applied exercises

Begin with the smallest observable program

The companion 00_starter.hy prints Hello, Hy!. In a disposable environment with Hy installed, run that file and record the Hy and Python versions. Hy compiles Lisp-like expressions into Python-compatible behavior; unfamiliar parentheses do not mean the runtime has different exception semantics from Python.

Transform data without hiding the steps

Create a scratch squares.hy and examine this expression:

(setv values [1 2 3])
(setv squares (list (map (fn [n] (* n n)) values)))
(print squares)
(print (sum squares))

The predicted Python-list-like values are 1, 4, 9 and the sum is 14. Trace from the innermost multiplication outward: fn creates the one-argument function, map applies it to each value, list materializes the iterator, and sum aggregates the result. Don't confuse an iterator with a fully evaluated list. Confirm exact printed formatting with the installed interpreter instead of treating a screenshot as a language specification.

Probe a real Python exception

Evaluate (print (/ 1 0)) in a separate file. The arithmetic must fail rather than printing a finite result. Save the actual traceback and identify the division expression, not the final print call, as the source. Use an exception handler only after reproducing the failure; otherwise it may conceal a wrongly typed input or an unintended operator.

Understand evaluation

Compare (list (map (fn [n] (+ n 1)) [1 2 3])) with a list comprehension written in ordinary Python. Explain the same logical input-output mapping despite different syntax. Next change the input list to empty, then to values containing strings, and classify whether the operation produces an empty result or an exception. A list of mixed types is a type-contract test, not evidence that Hy ignores types.

Independent extension

Implement a function that rejects negative scores and computes the sum of nonnegative squares, then test [], [0], [-1] and [2 3]. State whether you filter, reject or normalize invalid data; those choices are not interchangeable. Run in an isolated interpreter and report versions and actual diagnostics. Reference: https://hylang.org/

Hy — Focused mastery pathway

Foundation exercise

Create a Hy script importing a Python standard-library function and calling it from Lisp-style parenthesized forms.

Problem-solving exercise

Implement list mapping and a small predicate in Hy, comparing the emitted behavior to equivalent Python code. Distinguish quoted data from evaluated forms.

Independent project

Write a file summarizer that reads a disposable UTF-8 fixture and reports validation errors without modifying source files. Pin the Hy release because macros and syntax change.