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Using icas (Giac C++ REPL)

icas is the interactive C++ command-line interface shipped with upstream Giac. It lets you evaluate Giac expressions directly, without going through Giac.jl or Julia at all. This is especially useful for isolating whether a problem lives in Giac.jl (the Julia bindings) or in Giac itself (the upstream C++ library).

If a computation produces an unexpected result in Julia, reproducing it in icas answers one question cleanly:

Launching icas via GIAC_jll

GIAC_jll ships the icas binary alongside the shared library, so you do not need a system install of Giac.

julia> using GIAC_jll

julia> GIAC_jll.icas_path  # absolute path to the binary
"/home/user/.julia/artifacts/…/bin/icas"

julia> run(`$(GIAC_jll.icas())`)

GIAC_jll.icas() returns a Cmd with the environment correctly set up (library paths, etc.), which is why you should prefer it over calling icas_path directly.

Running icas from a shell (outside Julia)

Once you know the artifact path printed by GIAC_jll.icas_path, you can launch icas directly from a terminal without starting Julia every time. Copy the path and run it:

# Linux / macOS — path from GIAC_jll.icas_path
~/.julia/artifacts/ecf2beae5145924fc8d151fa5b57b85738c8367b/bin/icas
# Windows PowerShell
& "$HOME\.julia\artifacts\…\bin\icas.exe"

You only need Julia the first time, to discover the path. After that you can add the bin/ directory to your PATH (or make a shell alias) and invoke icas like any other command:

export PATH="$HOME/.julia/artifacts/ecf2beae5145924fc8d151fa5b57b85738c8367b/bin:$PATH"
icas
Note

The artifact hash in the path (ecf2be…) changes when GIAC_jll is updated. If icas suddenly fails to start after upgrading Giac.jl, re-run GIAC_jll.icas_path in Julia to get the new path.

A session looks like:

Welcome to giac readline interface, version 2.0.0
(c) 2002,2023 B. Parisse & others
…
Press CTRL and D simultaneously to finish session
Type ?commandname for help
0>> sin(2)
sin(2)
1>> sin(2.0)
0.909297426826
2>> evalf(sin(2))
0.909297426826

Press Ctrl-D to exit.

Reproducing a Giac.jl issue in icas

A typical triage workflow:

  1. Note the exact Giac expression Julia sends to the library. For most Giac.jl calls this is just the textual form of the command, e.g. integrate(sin(x)^2, x).
  2. Launch icas as above.
  3. Type the same expression at the >> prompt and compare the result with what Giac.jl returned.
  4. Include both outputs in your bug report.
Tip

If you are unsure what string Giac.jl actually sends to Giac, you can evaluate the expression in Julia and print it — the Giac string form is what icas will parse.

See also