Lig­a­tures in pro­gram­ming fonts—a mis­guided trend I was hop­ing would col­lapse un­der its own il­logic. But it per­sists. Let me save you some time—

Lig­a­tures in pro­gram­ming fonts are a ter­ri­ble idea.

And not be­cause I’m a purist or a grump. (Some days, but not to­day.) Pro­gram­ming code has spe­cial se­man­tic con­sid­er­a­tions. Lig­a­tures in pro­gram­ming fonts are likely to ei­ther mis­rep­re­sent the mean­ing of the code, or cause mis­cues among read­ers. So in the end, even if they’re cute, the risk of er­ror isn’t worth it.

First, what are lig­a­tures? Lig­a­tures are spe­cial char­ac­ters in a font that com­bine two (or more) trou­ble­some char­ac­ters into one. For in­stance, in ser­ifed text faces, the low­er­case f of­ten col­lides with the low­er­case i and l. To fix this, the fi and fl are of­ten com­bined into a sin­gle shape (what pros would call a glyph).

In this type de­signer’s opin­ion, a good lig­a­ture doesn’t draw at­ten­tion to it­self: it sim­ply re­solves what­ever col­li­sion would’ve hap­pened. Ide­ally, you don’t even no­tice it’s there. Con­versely, this is why I loathe the Th lig­a­ture that is the de­fault in many Adobe fonts: it re­solves noth­ing, and al­ways draws at­ten­tion to itself.

Lig­a­tures in pro­gram­ming fonts fol­low a sim­i­lar idea. But in­stead of fix­ing the odd trou­ble­some com­bi­na­tion, well-in­ten­tioned am­a­teur lig­a­tur­ists are adding dozens of new & strange lig­a­tures. For in­stance, these come from Fira Code, a heav­ily lig­a­tured spin­off of the open-source Fira Mono.

So what’s the prob­lem with pro­gram­ming ligatures?

1. They con­tra­dict Uni­code. Uni­code is a stan­dard­ized sys­tem—used by all mod­ern fonts—that iden­ti­fies each char­ac­ter uniquely. This way, soft­ware pro­grams don’t have to worry that things like the Greek let­ter Δ (= up­per­case Delta) might be stashed in some spe­cial place in the font. In­stead, Uni­code des­ig­nates a unique name and num­ber for each char­ac­ter, called a code point. If you have a Δ in your font, you as­so­ciate it with its des­ig­nated Uni­code code point, which is 0x0394. In ad­di­tion to al­pha­betic char­ac­ters, Uni­code as­signs code points to thou­sands of sym­bols (in­clud­ing emoji).

   The prob­lem? Many of the pro­gram­ming lig­a­tures shown above are eas­ily con­fused with ex­ist­ing Uni­code sym­bols. Sup­pose you’re look­ing at a code frag­ment that uses Uni­code char­ac­ters and see the sym­bol ≠. Are you look­ing at a != lig­a­ture that’s shap