Fusion Summon Choices

Before this lesson: read Fusion Procedures to distinguish a Fusion Monster's material requirement from the effect that summons it. Goal: explain each prompt, keep alternative material routes separate, and rebuild the legal choices when the effect resolves.

This lesson uses the pinned Utilities/procedure.lua and ordinary Scripts/ cards. Rush has a separate summon library.

The Player Makes More Than One Choice

Stage	Question the script answers	Does the player choose yet?
Activation check	Is at least one Fusion Monster summonable with a legal set of materials?	No. chk==0 only checks.
Resolution search	Which Fusion Monsters are summonable with the materials available now?	No. Build the current candidates first.
Monster selection	Which of those Fusion Monsters will be summoned?	Yes. Choose one monster.
Route selection	If both normal materials and Chain Material work for that monster, which route will be used?	Yes, when both routes exist.
Material selection	Which legal material set will summon that exact monster through that route?	Yes. Use its Fusion procedure.
Execution	Record the materials, move them through the chosen route, and summon.	Follow the committed choices.

Choosing a Fusion Monster at resolution does not make it a card targeted at activation. Polymerization (24094653) has a target callback, but that callback only checks feasibility and announces the summon. It does not call Duel.SelectTarget.

Begin With A Complete Helper-Based Spell

This is a complete tutorial script for a plain Fusion Spell using the local helper's normal material locations. Give it its own matching database ID and Normal Spell entry; the Fusion Monsters in the Extra Deck need their own material procedures.

local s,id=GetID()

function s.initial_effect(c)
    aux.AddFusionEffectProcUltimate(c,{})
end

aux.AddFusionEffectProcUltimate creates and registers an activation effect. Its target callback checks whether a summon exists. Its operation gathers materials again, builds the available Fusion Monster list, and handles the prompts above. The empty table uses the helper defaults; it does not mean that every Extra Deck monster or every group of monsters is legal.

Try it with two Fusion Monsters having different material requirements. Make one requirement impossible, then make both possible. The monster prompt should offer only the currently feasible choices. This is a useful first test before adding a custom material filter.

Read The Two Selection Calls

Here is the normal-route selection core from the Polymerization pattern, shown as a resolution fragment. e, tp, and chkf=tp belong to the operation; mg1 is the freshly filtered material pool; sg1 is the Group of Fusion Monsters already proven summonable through that pool.

Duel.Hint(HINT_SELECTMSG,tp,HINTMSG_SPSUMMON)
local selected=sg1:Select(tp,1,1,nil)
local tc=selected:GetFirst()
if not tc then return end

local materials=Duel.SelectFusionMaterial(tp,tc,mg1,nil,chkf)
tc:SetMaterial(materials)

Group.Select chooses the destination monster. Duel.SelectFusionMaterial then invokes that monster's registered material procedure. Selecting two arbitrary cards with Group.Select(tp,2,2,nil) would check only a quantity, not named material slots, substitute rules, or additional group restrictions.

The fourth argument to SelectFusionMaterial is a required material Card, or nil. Surprise Fusion (35167375) passes its changed monster there so that it must be used. The same required card must appear in the corresponding CheckFusionMaterial check.

This is not a cancellation flag

In this core, the optional sixth argument of Duel.SelectFusionMaterial is not_material. It is not a generic “allow Cancel” argument. The normal Polymerization monster choice uses Select(tp,1,1,nil); do not add an exit option simply because another selection API accepts a Boolean.

Chain Material Is An Alternative Route

Polymerization and FusionEffectUltimateOperation keep these Groups separate:

Group	Meaning
mg1	Materials for the normal route.
sg1	Fusion Monsters legal through mg1.
mg2	Materials supplied by the Chain Material effect.
sg2	Fusion Monsters legal through mg2 and that effect's filter.

They merge sg1 and sg2, so a monster appears in the choice list when either route can summon it. They do not merge mg1 and mg2 into a material pool that can mix the two permissions.

After the player picks tc, the route follows this table:

tc is legal through	What happens
Normal route only	Select from mg1 and use the normal material movement.
Chain Material only	Select from mg2 and call the Chain Material operation.
Both	Ask Duel.SelectYesNo(tp,ce:GetDescription()) whether to use Chain Material.

Here ce is the Effect returned by Duel.GetChainMaterial(tp). Its operation owns its material handling and summon behavior. Moving mg2 materials yourself and then calling that operation can perform the action twice or bypass its restrictions.

Optional Summoning Has Its Own Decision Point

Surprise Fusion first changes a targeted monster's Race/Attribute, then offers its optional Fusion Summon if a legal route exists. The bounded choice pattern is:

-- After the earlier action and a fresh legal-Fusion check:
if canFusion and Duel.SelectYesNo(tp,aux.Stringid(id,2)) then
    -- Rebuild the available Fusion Monsters, then choose monster and materials.
end

canFusion represents the result of that check, not just “there is a Fusion Monster in the Extra Deck.” The full source also verifies that the earlier change happened and that the required monster is still usable. The player declines the optional summon at this point; that differs from backing out of a material selection to revise an earlier choice.

Do not copy the yes/no prompt into mandatory Polymerization behavior. A prompt should implement an actual choice in the card text.

Additional Locations Also Need Movement Rules

For a tutorial variant whose text explicitly allows GY materials, the helper configuration can be:

local s,id=GetID()

function s.initial_effect(c)
    aux.AddFusionEffectProcUltimate(c,{
        mat_location=LOCATION_HAND+LOCATION_MZONE+LOCATION_GRAVE
    })
end

In this snapshot, the helper defaults GY material to an IsAbleToRemove filter and banishment through aux.FMaterialRemove. The ordinary material operation is sending to the GY. Therefore, adding LOCATION_GRAVE does not mean “send the GY cards to the GY again.” Confirm those defaults against your text before using this variant.

For different movement, change both the relevant filter and operation. Mementotlan Fusion (66518509) is a real manual example: conditional GY access, IsAbleToDeck on those materials, then separate movement for selected GY cards and ordinary materials. Its destruction flag also gates GY access, and the material set must contain a Memento monster. Do not copy just the Group merge and omit those restrictions.

Recheck Before Committing The Choice

At resolution, reconstruct the material pool and destination candidates. An opponent's earlier Chain Link can remove a material, change control, alter immunity, or change summon restrictions. A Group remembered at activation is not a fresh legality check.

For custom group restrictions, the main guide's aux.FCheckAdditional pattern must cover both feasibility checks and the final material selection. Clear the temporary hook afterward, including before returning from a new failure branch.

Keep Duel.BreakEffect() only where the source-backed timing sequence needs it. It separates effect timing; it is not an animation delay or an opportunity to rewrite choices already committed by the resolving effect.

Practice And Review

Test state	What to inspect
Two legal Fusion Monsters	Both offered; picking one changes which material sets are legal.
One apparently suitable monster with no legal material set	It is excluded from the monster prompt.
A required material leaves before resolution	The required-material route fails instead of substituting an unrelated card.
Only Chain Material works	Its materials and operation are used without requiring a normal route.
Both routes work	The route choice appears; materials never mix across the two routes.
GY access is conditional	Removing that condition removes GY materials from both checking and selection.
Optional summon declined	The optional branch is skipped; earlier actions remain resolved.

Continue with Ritual Summon Choices to see a different kind of cancellation: returning from material selection to monster selection.

Source Trail

• Utilities/procedure.lua: AddFusionEffectProcUltimate, FusionEffectUltimateTarget, FusionEffectUltimateOperation.
• Scripts/c24094653.lua: normal route versus Chain Material.
• Scripts/c35167375.lua: optional summon and required material.
• Scripts/c66518509.lua: conditional GY materials and different movement.
• Companion ygopro-core/libduel.cpp: duel_select_fusion_material argument handling.